Guidance Notes for Repairing Marine Propellers(Second Edition)

July 2002Nippon Kaiji Kyokai

Introduction

For repairing the surface defects found during the manufacture or the damaged part caused by an accident,propellers may be subjected to repair welding. As a general rule, when defects or cracks are found on a

propeller, they should be repaired by grinding-off without welding. The repairs by welding are thereforerestricted to the case where the intended repairs can increase the strength or technical reliability of thepropeller.

As for the repairing procedure of propellers, in 1983, “Guidance for Repairing Marine Propellers” wasprovided by the Society as a technical reference for the surveyors of the Society. Subsequently, a part of the

guidance was incorporated into the Rules and Regulations for the Construction and Classification of SteelShips and the Guidance (hereinafter referred to as “Rules” and “Guidance” respectively), and further revisionsbased on IACS UR W24 (1997) were made to the Rules and the Guidance in 2000. However, for lack of

detailed information on the repairs in the present Rules, the Guidance Notes have been prepared as the 2ndedition, which is consistent with the present Rules.

The Guidance Notes giving the interpretations and reference materials of the rules for repairing propellers areavailable for reference when field surveyors judge whether the applied repairing procedures are acceptable ornot. However, It must be borne in mind that sufficient discussions be made between the parties concerned for

each case involved.

Guidance Notes for Repairing Marine Propellers (Second Edition)

1. Application 1

2. Propeller materials 12.1 Chemical compositions and mechanical properties 1

2.2 Zinc equivalent 13. Severity zones for repairs 24. Repair welding 3

4.1 General notes on repair welding 34.2 Welding procedures 4

4.3 Edge preparation 55. Straightening 6

5.1 General notes for straightening 6

5.2 Hot straightening 65.3 Cold straightening 6

EXPLANATORY NOTES

1. Application 7

2. Propeller materials 7(1) High strength brass casting (KHBsC1) 7

(a) Equilibrium diagram and micro-structure 7(b) Zinc equivalent 7

(2) Aluminium bronze casting (KAlBC3) 8

(a) Equilibrium diagram and micro-structure 8(b) Weldability and lead content 8

3. Severity zones for repairs 10(1) Revision of severity zones 10(2) Damage to HSP 10

(3) Welding on a propeller boss 114. Repair welding 12

4.1 General notes on repair welding 124.2 Welding procedures 12

(1) Welding methods 12

(2) Filler metals 13(3) Preheating and post-weld heat treatment 13

(4) Stress relief and thermal brittleness 14(5) Temperature gradient 15

4.3 Edge preparation 16

5. Straightening 175.1 General notes for straightening 17

5.2 Hot straightening 175.3 Cold straightening 17

REFERENCE MATERIALS ”CHARACTERISTICS OF WELDED JOINTS” 18

REFERENCES 21

APPENDIX “WELDERS QUALIFICATIONS TEST (March 1993)” 22

- -1

1. Application

The Guidance Notes apply to cases where repairs by welding or straightening are carried out on propellers

made of high strength brass casting (KHBsC1) or aluminium bronze casting (KAlBC3) during themanufacture or in post-delivery service.

2. Propeller materials

2.1 Chemical compositions and mechanical properties (7.2.3 and 7.2.5, Part K of the Rules)

The Rule-required chemical compositions and mechanical properties of high strength brass casting(KHBsC1) and aluminium bronze casting (KAlBC3) are given in Tables 1 and 2.

Table 1 Chemical compositions (% )

Material Cu Al Mn Zn Fe Ni Sn Pb

High strength brasscasting (KHBsC1) 52 - 62 0.5 - 3.0 0.5 - 4.0 35 - 40 0.5 - 2.5 1.0 max. 0.1 - 1.5 0.5 max.

Aluminium bronzecasting (KAlBC3) 77 - 82 7.0 - 11 0.5 - 4.0 1.0 max. 2.0 - 6.0 3.0 - 6.0 0.1 max. 0.03 max*

*Note :In the case of aluminium bronze casting, its elongation falls significantly with low melting point when the lead content ofimpurities increases: thus cracks are liable to occur during the process of welding or hot straightening. Accordingly, the leadcontent is restricted to 0.03% considering possible post-manufacture reconditioning by welding or by hot straightening.

Table 2 Mechanical properties (separate casting)

Material Proof stress(N/mm2)

Tensile strength(N/mm2)

Elongation (L=5d)(%)

High strength brass casting(KHBsC1)

175 min. 440 min. 20 min.

Aluminium bronze casting(KAlBC3)

245 min. 590 min. 16 min.

Note :(1) The requirements specified in this Table apply to specimens cut from separately-cast samples,

where specimens cut from propeller casting itself, the requirements are to be deemed appropriateby the Society.

(2) The requirements concerning proof stress apply to cases where proof stress is required by theSociety in relation with design.

2.2 Zinc equivalent (7.2.3, Part K of the Rules)

The micro-structure of high strength brass casting comprises α+β phase. The volume of β-phase in thisstructure increases with the increase of the zinc equivalent. As the volume of β-phase increases, the ductilityand resistance to corrosion fatigue of propeller material decrease and the weldability also deteriorates.

Therefore, when manufacturing the propeller, the zinc equivalent as defined in the following equation isrestricted to within 45% assuming possibilities of conducting repairs of propellers after their manufacture:

Zinc equivalent (%) = 100 - ----------------------

where

A = Sn + 5Al - 0.5Mn - 0.1Fe - 2.3Ni (%)

If the proportion of α-phase determined from an average of five counts is not less than 25% in a sample takenfrom the tensile test specimen, the above zinc equivalent is recognized to be satisfied.

100 x Cu (%)100 + A

- -2

3. Severity zones for repairs (7.2.10-2(1), Part K of the Rules)

As shown in Fig.1, the surfaces on the pressure side and suction side of the blade are divided into the three

zones A, B and C. The propriety of the repairing method that can be applied to each of these zones andpropeller boss is given in Table 3.

Note:

(1) R is the radius of the propeller, Cr is the chord length at any radius.

(2) Highly skewed propeller is a propeller with a skew angle exceeding 25o.

(3) The boss area of a integrally cast propeller is regarded as zone C.

(4) The zones for non-destructive inspection in the root areas of the controllable pitch or build up propeller

blades and controllable pitch propeller bosses are to be deemed appropriate by the Society.

(5) Where stress distribution on propeller blade surfaces is estimated in detail, the non-destructive

inspection zones different from those shown in this figure may be applied provided the Society’s

approval.

(6) * Where the propeller boss radius (Rb) exceeds 0.27R, the boundary is to be increased to 1.5Rb.

Fig.1 Severity zones for repairs

0.15Cr

Fillet

C

B

0.7R

Pressure side

Leading edge

A

0.4R*

0.2Cr

C

B

Suction side

0.7RFillet

(a) Propeller other than highly skewed propeller

BA

0.15Cr

Suction side

Fillet

0.9R

Leading edge

Pressure side

Fillet0.4R*

0.7R

0.9R

B

A0.3Cr

0.5Cr

(b) Highly skewed propeller

- -3

Table 3 Propriety of repair methodsZones for repairs Classification

Repair method A B CPropeller boss

Building-up of defects by weldingBuilding-up of eroded parts by welding Unacceptable Acceptable Acceptable -

Chipping of blowholes and weldingChipping of cracks and welding Unacceptable Acceptable Acceptable Acceptable

(see explanatory notes)

Cut and butt welding UnacceptableAcceptable

only for leading ortrailing edge of blade

Acceptable -

Hot straightening(including a pitch modification) Acceptable Acceptable Acceptable -

Cold straightening UnacceptableAcceptable

only for leading ortrailing edge of blade

Acceptable -

4. Repair welding

4.1 General notes on repair welding (7.2.10-1, -2(2) and -2(3), Part K of the Rules)

(1) The work-site for repair welding shall be clean and free of harmful dusts, dirt, metal powder andexcessive humidity. Besides, there is a necessity of building a satisfactory shelter for protection against

wind and rain.

(2) Repair welding during manufacturing a propeller is permitted only when it is recognized as technically

necessary in comparison with removing casting defects by grinding.

(3) In principle, the work shall be done after removing the propeller from shaft and welding shall be done indown-hand (flat) position.

(4) The welders are to have sufficient technical knowledge and experience with regard to the weldingprocess to be executed. For instance, it is assumed that those welders who have passed appropriate

technical competence examination like “Welders Qualification Test” as described in the Appendix areconsidered to have sufficient technical qualifications.

(5) The welding operator shall carry out the welding procedure test to check if the welding process to be

executed serves the purpose or not.

(6) When carrying out repairs by welding, the casting defects or cracks in the damaged part of the propeller

are to be removed completely before welding. Then, a detailed inspection such as a dye penetrant testis to be carried out to confirm that no defects or cracks remain in the part.

(7) For details with regard to the welding procedure, see section 4.2.

(8) When the welding operation is terminated, the uneven reinforcement of weld is to be chipped off to give ita smooth finish. Once this is done, a detailed inspection such as a dye penetrant test is to be carried out

to confirm that no defects or cracks remain in the welded part.

(9) When the necessary stress relieving heat treatment is terminated, the inspection mentioned in item (8)above shall also be carried out after the heat treatment.

(10) The welding operator shall keep records of defective or cracked locations, their dimensions, results ofmicro-structure tests of damaged parts, welding conditions and so forth.

- -4

4.2 Welding procedures (7.2.10-2(2), Part K of the Rules, 7.2.10, Part K of the Guidance)

The procedure of repair welding is to comply with Tables 4 and 5. The welding conditions are shown in

Table 6.

Table 4 Welding procedures Material

Weldingcondition

KHBsC1 KAlBC3 Remarks

Welding method MIG weldingTIG welding

MIG weldingTIG welding -

Filler metals Aluminium bronzeErosion-resistant alloy

Aluminium bronzeErosion-resistant alloy

Aluminium bronze filler metals toconform to JIS Z 3341 YCuAlNiB orAWS A5.7 ERCuAl-A2 Code

PreheatTemperature (°C)

150 min. 50 min. -

Stress relieftemperature (°C)

350 - 500In zone B and boss, it is desirable tohave stress relieving heat treatmentwithin a range from 450 to 550 oC.

For Soaking times, see Table 5

Interpasstemperature (°C)

300 max. 250 max. -

Note:(1) Peening may be carried out on the welds or heat affected zones for each layer of bead excluding the initial path of weld.(2) When preheating or stress relieving heat treatment is carried out, temperature measurements are to be made by a

thermocouple or temperature chalk in order to verify that the temperature of the object is within the specified range.(3) Care must be taken so as not to make the temperature gradient of the propeller surface excessively large.(4) After welding or stress relieving heat treatment, the heated area and in the vicinity shall be cooled gradually.

Table 5 Soaking times for stress relief heat treatmentKHBsC1 KAlBC3Stress relief

temperature(°C)

Hours per 25mmthickness

Maximum soakingtimes (h)

Hours per 25mmthickness

Maximum soakingtimes (h)

350 5 15 - -

400 1 5 - -

450 1/2 2 5 15

500 1/4 1 1 5

550 - - 1/2 2

Table 6 Welding Conditions Welding methodWelding condition

MIG welding TIG welding

Polarity DC, reverse polarity AC, HF

Diameter of filler rod (or wire) (mm φ) 1.2 - 1.6 1.6 - 5.0

Electrode diameter (mm φ) - 2 - 4

Welding current (A) 200 - 350 100 - 350

Flow rate of Argon (lit./min) 20 - 30 15 - 25

- -5

4.3 Edge preparation

(1) Shapes of groove for building-up welding are to be as shown in Figs.2 and 3.

Fig.2 Shape of groove for building-up Fig.3 Shape of groove for building-up welding (eroded portion) welding (defective portion)

(2) H-shape groove for butt welding is to be as shown in Fig.4. However, if the blade thickness at thewelding part is 30mm or less, V-shape groove is also acceptable.

Fig.4 Shape of groove for butt welding

(3) For the welding after chipping blowholes or cracks, the shape of groove can be determined arbitrarilyaccording to the shape of the blowholes or cracks. However, the torch shall reach properly the bottomsurface and the shape of groove shall be made in such a way that the welding operation can be carried

out easily. For instance, if a shape of groove as shown in Fig. 5 is assumed, the bevel angle shall beset to 30o or more and the corner radius of 6R or more shall be provided at the bottom surface.

Fig.5 Shape of groove for filling blowholes and crack by welding after chipping

30o or more 30o or more

6R 6R

30o or more

Backing strip

60o or more

2/3 T

60o or more

TBasemetalPiece to be joined

6R ormore

Carry out sufficient back chipping andleave 6R at the bottom corner.

30o or more 30o or more

6R ormore

- -6

5. Straightening

5.1 General notes for straightening (7.2.8-2, Part K of the Rules)

(1) Before and after execution of straightening operation, a dye penetrant test is to be carried out to confirmthat there are no harmful casting defects or cracks. Besides, when executing the stress relieving heat

treatment, the same test is to be conducted after this operation.

(2) When reconditioning work by twisting is done for a pitch modification at the root portion of a propellerblade, the actual pitch shall be measured to determine the amount of pitch alteration before starting the

work.

(3) For details of hot straightening, see section 5.2.

(4) For details of cold straightening, see section 5.3.

5.2 Hot straightening (7.2.8-2(2), Part K of the Rules)

The method of hot straightening is given in Table 7.

Table 7 Hot straightening conditions

Material

Conditions

High strength brass casting(KHBsC1)

Aluminium bronze casting(KAlBC3) Remarks

Straighteningtemperature

(°C)500 - 800 700 - 900

When heating is made with apropane gas torch or acetylenegas torch, care must be taken soas not to cause an extreme localheating due to the concentratedflame.

Stress relieftemperature

(°C)350 - 500

In zones A and B, it isdesirable to have stressreliving heat treatment withina range from 450 to 550 °C.

For soaking times, see Table 5.

Note:(1) When hot straightening or stress relieving heat treatment is carried out, temperature measurements are to be made by a

thermocouple or temperature chalk in order to verify that the temperature of the object is within the specified range.Care must also be taken so as not to make the temperature gradient of the propeller surface excessively large.

(2) After hot straightening or stress relieving heat treatment, the heated area and in the vicinity shall be cooled gradually.

5.3 Cold straightening (7.2.8-2(3), Part K of the Rules)

In principle, the cold straightening (straightening temperature is 200 °C or less) shall be done under staticload by hydraulic jack etc. Hammering or other impact load must not be applied except for slight straighteningof the propeller tips as well as leading or trailing edge of the blade.

With regard to stress relieving heat treatment, it is recommended that the requirements in the conditions ofTable 7 be applied.

- -7

EXPLANATORY NOTES1. Application

The Guidance Notes apply to cases where welding is done for filling blowholes during the manufacture ofpropellers or where repair welding or straightening of propellers damaged in an accident or when repairing

work is done on propeller blades for a pitch modification and so forth.As for the propeller material, only two types of materials, i.e., KHBsC1 and KAlBC3 are considered in theGuidance Notes. Other materials such as other copper alloy, cast iron and cast steel are excluded from the

scope of the Guidance Notes, because the use of these materials is rare and their repairing methods are alsodifferent.

2. Propeller materials

(1) High strength brass casting (KHBsC1)

(a) Equilibrium diagram and micro-structure

The equilibrium diagram of copper-zinc alloy is shown in Fig.6. In this alloy, the corrosion-resistant

characteristics of α-phase is superior. However, this is quite soft with a high degree of malleability. On theother hand, β-phase is hard with a high degree of tensile strength. However, it has quite a few drawbacks asit is susceptible to intergranular corrosion, stress corrosion crack, dezincification, etc.

Fig.6 Equilibrium diagram of copper-zinc alloy (1) Fig.7 Micro-structure of high strength brass

The high strength brass casting as a propeller material is manufactured by adding following compositions to

the copper-zinc alloy: manganese for increasing the hardness and tensile strength, iron and aluminium forrefining the grain size, and nickel and tin for improving the corrosion resisting characteristic. Fig.7 shows the

micro-structure of the material, which represents α+β phase. (2)

(b) Zinc equivalent

The relationship between the mechanical properties of high strength brass casting and the zinc equivalent is

shown in Fig.8. The results shown in the figure were obtained by controlling the cooling speed in threedifferent stages: slow cooling, ordinary cooling and quenching. In this test, small test specimens were

individually cast into the mould for obtaining the cooling speeds.(3)

As can be seen from the figure, the zinc equivalent exceeding the value of approximately 45% brings aboutan increase of the tensile strength and a large decrease of the elongation. As an effect of the elements other

(x 400)

- -8

than zinc on the structure, it is well known that with the increase of volumetric content of manganese, ironand nickel or with the decrease of tin and aluminium, the ratio of β-phase in the micro-structure decreases.

Regarding the distribution of α-phase in the micro-structure, if the extent of this distribution is found to be25% or more in a sample taken from the tensile test specimen, the material may possess almost the same

mechanical properties as in the case when the zinc equivalent is 45% or more.(4)

Slow cooling signifies the case where the melted metal ispoured into the sand mould and placed with the mould in anelectric furnace so that the temperature reaches the roomtemperature in 20 hours.

Normal cooling signifies the case where the melted metal ispoured into the sand mould and left in the atmosphere fornatural cooling.

Quenching signifies the case where a chill mould is used forcooling the metal object.

Fig.8 Relationship between zinc equivalent and

mechanical properties (3)

(2) Aluminium bronze casting (KAlBC3)

(a) Equilibrium diagram and micro-structure

The equilibrium diagram of copper-aluminium alloy is shown in Fig. 9. In this alloy, when the element

containing the composition of β-phase is cooled slowly down to 565oC or under, a brittle α+γ2 phase is formed,resulting in reductions of tensile strength, elongation and impact test values, and this features the so-called

slow cooling brittleness of aluminium bronze.(6)

As shown in Fig.10, the aluminium bronze casting as a propeller material is manufactured by adding nickeland iron to the copper-aluminium alloy in order to shift the phase containing γ2 to the high aluminium

containing side. The structure of the element manufactured in this process represents α+κ phase. Themicro-structure of aluminium bronze casting is shown in Fig.11.

(b) Weldability and lead content

What exerts the greatest influence on weldability of propellers is ductility. As one of impurities, propellermaterials contain lead that almost does not form any solid solution in the copper alloy and remains in the

intergranular boundary as it is. Especially in the case of aluminium bronze casting, the ductility is greatlyreduced in the presence of an excessive volume of lead. (7)

Besides, if the lead content is high, the melting point of the casting falls. In such an instance, this raises thepossibility of generating cracks during welding or hot straightening; therefore the lead content is limited to0.03% in the Rules and IACS UR W24.

- -9

Fig.9 Equilibrium diagram of copper-aluminium Fig.10 Equilibrium diagram of aluminium bronze alloy (5) added with 5%Ni and 5% Fe (5)

Fig.11 Micro-structure of aluminium bronze casting

(x 400)

- -10

3. Severity zones for repairs

(1) Revision of severity zones

In Fig.1, the surface of a propeller is divided into zones A, B and C in consideration of the stress distributiongenerated on the propeller surface and the degree of damage affecting a ship operation, which is based on

the Unified Requirements of IACS. In the Guidance for Repairing Marine Propellers of 1st edition, zones forConventional Propeller (CP) had already been described. At this time, the revision of severity zones hasbeen carried out including the case of Highly Skewed Propeller (HSP), which were prescribed in IACS UR

W24 (1997) and the Rules (2000).

In CP, the tensile stress acting on the propeller in service has the maximum near the thickest portion of the

root of the blade on the pressure side. On the other hand, in HSP, higher stresses are generated not onlynear the thickest portion of the root of the blade but also near the trailing edge of 0.6R as shown in Fig.12.Depending on the skew angle, the stress has the maximum at the trailing edge of the blade.(9) Therefore, the

repair welding at this high stress portion raises the possibility of causing fracture of the blade due to thetensile stress repeatedly acting on the blade as well as the residual stress after welding.

Fig.12 Relationship between skew angles and stress distributions of the blade (8)

(2) Damage to HSP

As for the damage to CP due to welding, there are plenty of examples of such a damage originating fromwelded portion at the root of the blade (10), and in most of these cases the damage occurred in a short period

of time. In the case of HSP, some of the damage reported are such that cracks originated near the trailingedge of 0.6R of the blade due to the contact with floating objects in reverse rotation of a propeller andpropagated toward the leading edge of 0.9R of the blade with resultant fracture of the blade.(13) Therefore, if

inappropriate repair welding is carried out at the trailing edge, the similar accident involving the fracture mayhappen. For this reason, especially in HSP, the region up to the trailing edge of 0.9R has been classified as a

zone A in which welding is generally not permitted.

When cracks are found on the trailing edge of the blade (zone A), they shall be chipped off and the chippedpart shall rather be left as it is in preference to being reconditioned by welding. In the accident involving

fracture of the blade at zone A as shown in Fig.13, the propeller is to be renewed in dock because welding isprohibited in this zone. However, if the fracture area is small, butt welding using new piece can be done in

accordance with note (5) of Fig.1, where the stress distribution on the propeller blade must be estimated indetail before the repair work.

- -11

Fig.13 Damage to the tip of a blade which is frequently seen in HSP (9)

(3) Welding on a propeller boss

In Table 3, chipping of blowholes or cracks followed by welding on a propeller boss is described as aacceptable method; however, the welding on a propeller boss is to be avoided as far as this is permitted.

When cracks due to stress corrosion etc. are found on a propeller boss, then mere grinding-off of the cracksis enough to repair the propeller without recourse to welding, and in most cases this is the best way. This isbecause there are difficulties in the stress relief in and around any welds on thick part of a propeller (that is

difficulties in keeping the propeller in the condition of stress relief temperature). The resultant residual stressdue to welding gives rise to higher possibility of initiating and propagating of new cracks than before the

repair. Judging from the past records of repairs, for removing of cracks, the propeller boss can be gouged outdeeply up to around 30% of its thickness while keeping the sufficient strength of boss. When cracks deeperthan this limit were found, the propeller is to be renewed (or such recommendation is to be described in the

survey report).

Regarding the severity zones for the root areas of blades of CPPs or build-up propellers and for bosses ofCPPs, which are described as remark (4) of Fig.1, refer to Fig. K7.2.8 -1 of the Guidance.

Sketch of Damage (missing)Blade-A, Pressure side

Container ship 36,500 GT, 20.1 kt 24,000 PS x 102 rpm

Propeller: Diameter 7,000mm, 4 blades Pitch ratio 0.8399 Skew angle 35 deg.

- -12

4. Repair welding

4.1 General notes on repair welding

(1) The general notes to be observed prior to welding are quoted from Section 2.4 JIS Z 3604Recommended Practice for Inert Gas Shielded Arc Welding.

(2) When manufacturing a propeller, casting defects in the form of pinholes (approximately 1 mm indiameter) are sometimes detected. However, there is no need of repairing such types of minor defectsunless they are found concentrated in a certain spot. If injurious defects that can generate cracks

resulted in the fracture of the propeller are detected, be sure to grind them off. However, it is alwaysadvisable to avoid repairs by welding as far as circumstances permit.

(3) As welding operation affects the quality of the job done, welding is generally done by removing thepropeller from the shaft and done in the down-hand (flat) position. This method ensures the reliability ofthe operation. However, for minor repairs of tip and edges of the blade, the work can be done without

removing the propeller from the shaft.

(4) The technical knowledge and experience of the welder can greatly affect the quality of the job performed.

Therefore, the competence of the welder is to be checked by appropriate method. As a method forconfirming the competence, there is the Welders Qualification Test determined by Japanese MarineEquipment Association.

(5) The welding procedure test is carried out to check if the welding procedure to be executed is appropriateor not. As a test for butt welding (7.2.10 (4)(a), Part K of the Guidance), the test method prescribed in IACS

UR W24 was introduced. When building-up welding or butt welding are intended for a repair method,this test method also serves as the welders qualification test. On the other hand, in the case of fillingblowholes by welding, a test for mold cavity welding (7.2.10 (4) (b), Part K of the Guidance) is to be carried

out in addition to the welders qualification test.However, for minor repairs of tip and edges of the blade, considering the condition of the damage and

the past record of the welder, the welders qualification test as well as the procedure test may be omittedif the Society’s surveyor agrees.

(6) As a preprocessing, it is highly important to fully eliminate any defects of the base metal. In order to

ensure that all defects have been completely removed, a close inspection such as dye penetrant test isrequired.

(7) See the description of item 4.2.

(8) In order to check if sound welding has been done or not, it is necessary to carry out a close inspectionsuch as dye penetrant test.

(9) As cracks may be generated by the temperature gradient due to partial heating, it is necessary toexecute the same inspection even after heat treatment.

(10) Naturally, it is necessary to keep a record of the location and dimensions of the defects or cracks as wellas the conditions relevant to the repair.

4.2 Welding procedures

(1) Welding methods

At present, the inert gas arc welding (MIG or TIG welding) is normally used in Japan for repair welding ofpropeller materials. Before, “casting with common metals” was used for reconditioning the defects in thebase metal of high strength brass casting. However, it had a drawback of causing inferior deposition because

of a high volume of volatile zinc contained in the base metal. On some occasions, the shielded metal arcwelding or CO2 gas shielded arc welding was used in the base metal of aluminium bronze casting. However,

the performance of the method was so bad that internal cracks could be easily generated. In short, it wasvery much inferior to the current inert gas arc welding.

- -13

Generally, when performing welding for high strength brass and aluminium bronze castings, it is alwaysadvisable to use inert gas arc welding method in which generation of oxides can be avoided. Especially in

high strength brass casting, if evaporation of zinc at low amperage can be restricted, an adequate repairwelding can be performed in terms of strength. Therefore, in the Guidance Notes, the welding operation are

restricted only to MIG or TIG welding. However, when minor repairs of tip and edges of the blade are done,any other welding methods such as gas welding etc. can be used.

(2) Filler metals

As a filler metal for inert gas arc welding, aluminium bronze which corresponds to JIS Z 3341 YCuAlNi B orAWS A 5.7 ERCuAl-A2 Standards can be generally used. The standard chemical compositions and

mechanical properties of filler metals are given in Table 8.

Table 8 Examples of code requirements for filler metals

Code Cu Si Mn P Pb Al Fe Ni Zn

* in totalJIS Z 3341YCuAlNi B Residue

0.1max

0.5 -3.0 *

*0.02max

7.0 -9.0

2.0 -5.0

0.5 -3.0

*0.10max 0.50 max

total other elementsAWS A5.7ERCuAl-A2 Residue

0.1 max - -

0.02max

8.5 -11.0

1.5max -

0.02max 0.05 max

After welding, the metallurgical composition of the high aluminium filler metal which corresponds to ERCuAl-A2 represents α+β phase. On the other hand, in the low aluminium filler metal which corresponds to YCuAlNiB, this represents the α+κ phase. When the base metal is aluminium bronze casting, the filler metals of the

common alloy family are sometimes used. In this case, if the aluminium content is made identical to the basemetal, the hardness of the boundary of weld becomes too high, thereby giving rise to the possibility of

causing welding cracks. (11) Therefore, it is advisable to use low aluminium filler metals in which thealuminium content is slightly lower than the base metal.

In the repair of eroded part of the propeller by building-up welding, high aluminium filler metals are used. The

metallurgical composition of the weld is very fine where the ratio of β-phase is higher. Unlike the filler metalsused for filling blowholes, the hardness of the weld metal of this alloy becomes identical to or higher than the

hardness of the heat affected zone.The chemical compositions and mechanical properties of the filler metals, which are in wide use now, aregiven in Table 9. In addition, Fig.14 shows the hardness distribution of KF alloy that is an example of

erosion-resistant welding materials for building-up.

(3) Preheating and post-weld heat treatment

When inert gas arc welding is performed without preheating, the propeller is partially overheated, resulting inthe evaporation of zinc. Especially in the base metal of high strength brass casting, the zinc is evaporatedvery easily. Therefore, the base metal must be preheated to the prescribed range of temperature before the

necessary welding is done. If the base metal is aluminium bronze casting, the change in metallurgicalcomposition due to evaporation of metal component is small; however, preheating is advisable for improving

the quality of the job to be performed.

Regarding post-weld heat treatment, in the base metal of high strength brass casting, there is a possibility ofgenerating stress corrosion cracks in the seawater. Therefore, in order to relieve the residual stress of the

weld, post-weld heat treatment must be done within the prescribed range of temperature. On the other hand,in the base metal of aluminium bronze casting, it is said that there is no need of stress relieving heat

treatment (13) as the base metal has a high resistance to the stress corrosion cracks. However, post-weld heattreatment can decrease the residual stress as much as possible; therefore, when welding is done in zone Bor on propeller boss, it is desirable to carry out stress relieving heat treatment as specified in the Guidance

- -14

Notes even for aluminium bronze casting.In addition, when building-up welding is done for the prevention of cavitation, the stress relieving heat

treatment is not necessary even for zone B regardless of propeller materials.

Table 9 Examples of chemical compositions and mechanical properties of filler metals

ApplicationTrade nameBrand name

(Code)Cu Al Fe Ni Mn Pb Si Others

Tensile strengthkgf/mm2

Elongation(%)

HardnessHB

Mitsubishi MetalMining Co., Ltd.

NW-5(YCuAlNi B)

Residue 7.60 3.47 1.00 0.92 0.001 - - 56 55 129

Nippon Oils and FatsCo., Ltd.MG860

(YCuAlNi B)

Residue 8.33 1.50 1.11 0.57 … - - 63 26 -

Building-up ofchipped areaby welding

Butt weldingAmpco (USA)

Ampco rod #10(ERCuAl-A2)

Residue 9.62 1.22 - 0.03 0.002 - - 60 24 133

Hitach Shipbuilding &Engineering Co., Ltd

HZ AlloyResidue 8.32 0.11 - - 0.001 -

Co0.93 81 15 232

Building-up oferoded portionby welding Kobe Steel Ltd.

KFAlloyResidue 8.97 1.84 1.95 9.06 0.001 -

Co1.08 90 21 244

Fig.14 Hardness distribution of KF alloy (12)

(4) Stress relief and thermal brittleness

As a method of relieving the residual stress of the weld, peening as well as heat treatment is effective. Fig.15shows an example of change in residual stress when the heat treatment and peening conditions are alteredin the base metal of aluminium bronze. As can be seen from this figure, in the stress relief temperatures and

soaking times of 350 °C x 3 hrs, 400 °C x 3 hrs and 600 °C x 3 hrs, the residual stresses are relieved from35% to 40%, 50% to 60% and 70% to 75% respectively in comparison with the non-processed base metals.

If the soaking time is made longer even at the same temperature, the achievement of relieving the stressbecomes greater. As an effect of peening, when peening is done both for the weld and heat affected zone ofthe base metal, the result of the stress relief is great.

- -15

Fig.15 Measured residual stresses under various Fig.16 Effects of heat treatment on heat treatment and peening condition (14) mechanical properties (14)

When the above heat treatment is done for aluminium bronze casting, the mechanical properties of the metal

hardly change by heating up to the temperature of 450 °C as shown in Fig.16. However, as the heatingtemperature is raised to 450 °C or above, the hardness of the metal rises with the decrease of the elongation.

This is due to the precipitation of κ-phase in α-phase of the base metal caused by re-heating. Therefore,considering the reduction of elongation of the base metal, it is advisable not to heat the metal exceeding thetemperature of 550 °C, although the heating for a longer period of time in the relatively high temperature

range can eliminate the residual stress effectively.

Besides, it is known that aluminium bronze casting shows the thermal brittleness in the vicinity of 350 °C,

resulting in the reduction of elongation. Accordingly, as there is a possibility of generating cracks in this low-temperature heat treatment process, this temperature range shall also be avoided. Considering the above, inthe Guidance Notes, we have recommended that the post-weld heating temperature of aluminium bronze

casting be kept within the range of 450 °C to 550 °C.Similarly, in consideration of the thermal brittleness of the copper alloy, we have recommended that the

interpass temperature of high strength brass casting be kept at 300 °C or below and of aluminium bronzecasting at 250 °C or below.

(5) Temperature gradient

When preheating or relieving stress by heat treatment, there is a possibility of generating cracks due tothermal stress; therefore, care must be so taken that the temperature gradient of the surface of the propeller

does not become great. If the propeller is cooled abruptly following the welding or stress relieving heattreatment, a large temperature difference is caused between the surface and internal structure of thepropeller, giving rise to the possibility of generating cracks due to the effects of thermal stress. Therefore, the

vicinity of the heated surface is to be cooled slowly. In IACS UR W24, as a standard, it is prescribed that thecooling rate after any stress relieving heat treatment shall not exceed 50 °C/hr until the temperature of 200 °C

is reached.

- -16

4.3 Edge preparation

In all welding repairs including blowhole filling by welding, building-up welding, butt welding, etc., the welding

grooves shall be prepared in such a manner that will allow a good fusion of the groove bottom by penetratingthe torch to the bottom. In the Guidance Notes, for all possible cases, the groove angle was set at 60 deg. or

more (bevel angle was set at 30 deg. or more), and a radius on the bottom was set at 6R or more.

The measurement results of the residual stresses in each shape of groove are shown below for reference. Ingeneral, the residual stress generated by welding varies depending on the constraints imposed by the base

metal or volume of weld.

One example of measured residual stress distribution in plug welding is shown in Fig.17. In this welding, an

extremely large residual stress is generated in the heat-affected zone of the metal, since all the periphery ofthe weld is completely constrained by the base metal. As an effect of volume of weld, when the plug area issmaller, the residual stress in the heat-affected zone of the metal is greater. When the plug area is large, the

fall of the cooling rate enlarges the stress relieving effect resulting in the decrease of the residual stress. And,when a taper is provided at the opening of the groove, the overall residual stress in each portion can

generally be reduced.

In butt welding, the magnitude of the residual stress varies depending on the plate thickness and the shapeof groove as shown in Fig.18. If welding is made in a thin plate, the base metal is deformed easily during

solidification and contraction of the weld metal, which consequently reduces the residual stress withdecreasing the thermal stress. Contrary to this, if the plate is thick, the constraining force of the base metal

increases resulting in the increase of residual stress.

Fig.17 Measured residual stress in plug Fig.18 Measured residual stress in butt

welding (14) welding (14)

- -17

5. Straightening

5.1 General notes on straightening

(1) As a preprocessing, a close inspection such as dye penetrant test is required, because it is highlyimportant to confirm if there is any casting defect or crack which may cause further cracks during the

straightening process.

(2) In the manufacturing process, there can be a certain amount of pitch error in the propeller. When a pitchmodification is done, the actual pitch is to be checked prior to the work; otherwise, the propeller may

have an exceeding pitch beyond the predetermined pitch modification. Although the allowable amount ofpitch modification is determined according to the diameter of the propeller, number of blades, expanded

area and so forth, it is generally agreed that the amount of pitch modification (in percentage) decreaseswith the increase of propeller diameter. According to the records of propeller manufacturers, for example,if a propeller is made of high strength brass casting and its diameter is 5 m, the pitch modification of up

to approximately 7% is carried out.

In the case of propellers made of high manganese aluminium bronze castings, which are the material

not within the coverage of the Guidance Notes, there are several examples of blade fractures followingpropeller pitch alteration; therefore, this repair method shall not be adopted.

(3) See the description of Section 5.2.

(4) See the description of Section 5.3.

5.2 Hot straightening

The straightening temperature is to be determined in consideration of thermal brittleness peculiar to thecopper alloy materials as well as the working conditions.

Generally, the residual stress generated by straightening does not become as large as that generated bywelding. However, in the case of high strength brass casting, there is a possibility of generating stress

corrosion cracks; therefore, it is necessary to perform the stress relieving heat treatment. In the case ofaluminium bronze casting, it has greater resistance against stress corrosion cracks, and hence, the heattreatment was given merely as an advice.

5.3 Cold straightening

The residual stress generated after the cold straightening is even larger than that generated after the hotstraightening. Therefore, especially in the case that the base metal is high strength brass casting, there is anabsolute necessity of performing stress relieving heat treatment.

- -18

REFERENCE MATERIALS”CHARACTERISTICS OF WELDED JOINTS”

Welded joints generally have the following characteristics:

(i) Metallurgical structure

The examples of micro-structures of welded joints using the high strength brass and aluminium bronze

castings as base metals are shown in Figs.20 and 21 respectively, where the micro-structures arephotographed as shown in Fig.19. In both cases, the aluminium bronze corresponding to YCuAlNi B (JIS

Z3341) Standard was used as the material of filler metals. The stress relieving heat treatment was conductedunder the condition of 370 °C x 2 hrs only in the base metal of high strength brass casting.Generally, when the high aluminium filler metals corresponding to AWS A5.7 ERCuAl-A2 Standard are used,

the metallurgical structure of the weld metal represents α+β phase, and when low aluminium filler metalscorresponding to JIS Z3341 YCuAlNi B Standard are used, this represents α+κ phase.

(11) (15)

The metallurgical structure of the heat-affected zone is very fine because of quick heating and cooling.In the base metal of high strength brass casting, β-phase of the micro-structure which is stable in the hightemperature range breaks into α+β phase by cooling. However, In the base metal of aluminium bronze

casting, β-phase remains unchanged while the fine κ-phase appears once again.

Fig.19 Photograph of micro-structure

Fig.20 Micro-structure of weld-base metal boundary (Base metal: KHBsC1, plate thickness: 25mm)

Fig.21 Micro-structure of weld-base metal boundary (Base metal: KAlBC3, plate thickness: 25mm)

Base metal Heat affected zone Weld

Base metal Heat affected zone Weld

Direction in which photo was taken

- -19

(ii) Tensile strength

Tables 10 and 11 show the results of tensile tests on butt welding using high strength brass and aluminium

bronze castings as base metals. In both cases, the aluminium bronze corresponding to ERCuAl-A2 Standardwas used as filler metals. The stress relieving heat treatment was conducted only in the base metal of high

strength brass casting with the following conditions: 360 °C x 3 hrs in test specimens A to D and 370 °C x 1hrs in test specimens E and F.

Table 10 Tensile strength of welded joints (Base metal: KHBsC1)

Base metal Welded jointTest

specimen Tensilestrength (A)(kgf/mm2)

Elongation(%)

Platethickness

(mm)

Weldingmethod Tensile

strength (B)(kgf/mm2)

(A) / (B)(%)

Location offracture

A 44.6 82.9 Weld

B53.8 37.5 38.0 TIG

45.5 84.6 Weld

C 49.6 91.5 HAZ

D54.2 39.8 38.0 TIG

51.7 95.4 HAZ

E 45.1 88.4 HAZ

F51.0 40.0 25.0 MIG

46.5 91.2 HAZ

Table 11 Tensile strength of welded joints (Base metal: KAlBC3)

Base metal Welded jointTest

specimen Tensilestrength (A)(kgf/mm2)

Elongation(%)

Platethickness

(mm)

Weldingmethod Tensile

strength (B)(kgf/mm2)

(A) / (B)(%)

Location offracture

A 60.3 88.9 Base metal

B67.8 30.0 38.0 TIG

61.0 90.0 HAZ

C 57.5 91.2 Base metal

D70.8 31.0 38.0 TIG

64.1 90.5 HAZ

E 62.4 86.7 Weld

F72.0 20.0 25.0 MIG

64.9 90.1 Weld

- -20

(iii) Hardness

The results of hardness tests of the welded joints with high strength brass and aluminium bronze castings as

base metals are shown in Figs.22 and 23 respectively. The test specimens used were the same as thoseshown in above (i).

In Fig.22, the hardness of weld metal is slightly higher than that of the base metal, because aluminiumbronze filler metals were used in the welding of base metal of high strength brass casting. In the heat affectedzone, the metallurgical structure of the metal is very fine due to the effect of quick heating and cooling

process, and the hardness is higher than that of the weld metal due to the increase of the ratio of β-phase inthe micro-structure.

In Fig.23, unlike Fig.22, the hardness of the weld metal is lower than that of the base metal, becausealuminium, iron and nickel contents of the filler metal used were lower than in the case of the base metal. Inthe heat affected zone, due to rapid cooling, β-phase in the micro-structure remains unchanged while κ-

phase reappears; consequently, the hardness in this zone becomes higher due to the tempering effect.

Fig.22 Hardness of welded joint Fig.23 Hardness of welded joint (Base metal: KHBsC1, plate thickness: 25mm) (Base metal: KAlBC3, plate thickness: 25mm)

- -21

REFERENCES

(1) Albert G. Guy: “Physical Metallurgy for Engineers,” 1962(2) J. Arai: “Propeller Materials,” Bulletin of Marine Engineering Society in Japan, Vol.12, No.1 (1977), p.11

(in Japanese)(3) S. Kondo: “On Propeller Materials,” Journal of Nippon Kaiji Kyokai, Vol.58 (1959), p.48 (in Japanese)(4) IACS UR W24: “Cast Copper Alloy Propellers ” (1997)

(5) P. Brezina: “Heat treatment of Complex Aluminium Bronze, ” International Metals Reviews, 1982,Vol.27, No.2, p.77

(6) M. Sugiyama: “The Properties and Application of Aluminium Bronze Castings,” Journal of the JapanSociety of Mechanical Engineers, Vol.66, No.534 (1963), p.61 (in Japanese)

(7) M. Kanamori and S. Ueda: “Effects of Additive Elements on Copper, Aluminium, Nickel and Ferrous

Family Alloy Castings,” Journal of the Japan Institute of Metals , Vol.24 (1960), p.209 (in Japanese)(8) T. Sasajima: “Dynamic Blade Stress on Marine Propellers Operating in Wake of Ship’s Hull,” Mitsubishi

Technical Bulletin, No.181 (1988)(9) S. Ryo: “Review of Damage to Highly Skewed Propellers,” Journal of Nippon Kaiji Kyokai, No.225

(1993), p.16 (in Japanese)

(10) H. Kume: “Review of Propeller Blades Failures,” Journal of Nippon Kaiji Kyokai, Vol.135 (1972),p.160 (in Japanese)

(11) Aluminium Bronze Committee of the Japan Society for the Promotion of Science: “AluminiumBronze,” 1967 (in Japanese)

(12) Kobe Steel, Ltd.: Catalogue on KF Alloy for Marine Propellers (in Japanese)

(13) Japanese Marine Equipment Association: ”Standards for Repairing Propellers during ManufactureSM A277,” 1993 (in Japanese)

(14) I. Nakano and W. Ishizu: “Residual Stress and Stress Corrosion Cracks of Aluminium Bronze WeldingMaterials,” Research and Development, Kobe Steel Engineering Reports, Vol.26, No.4 (1976), p.59 (inJapanese)

(15) I. Nakano and S. Oshima: “Latest Propeller Repair Techniques,” Metals & Technology, Vol.50, No.11(1980), p.58 (in Japanese)

- -22

APPENDIXWELDERS QUALIFICATIONS TEST (March 1993)

(Japanese Marine Equipment Association)

1. PurposeThe purpose of the test is to examine the ability of welders who carry out repairs of propellers made of copperalloys by welding.

2. Test samples

One set of sand moulds of two pieces with dimensions of 280 x 125 x 25 mm shall be prepared as the testsamples with the chemical composition nearly compatible with that of the propeller.

3. Edge preparationSee Fig.1.

4. Preparation of test specimensSee Figs. 2 and 3.

5. Acceptance criteria

Acceptance is based on the results of the following two tests.

Test item Base metal Test value(welded portion) Acceptance Remarks

Radiographictest by X-ray For all materials JIS Grade 3 or upward * Acceptable For all welds

High strength brass(HBsC1,HBsC2)

Tensile strength390 N/mm2 or above Acceptable For all three test

specimensTensile test

Aluminium bronze(ALBC3)

Tensile strength540 N/mm2 or above Acceptable For all three test

specimens

* JIS Z 3104 shall apply mutatis mutandis.

6. Others

(1) The bending test, hardness test as well as macro and micro tests are as reference items.

(2) This test is performed at each time when the filler metal or welding process used is changed.

- -23

Fig.1 Procedure for edge preparation Fig.2 Preparation of test specimens

Fig.3 Dimensions of test specimens