08 how to repair surface defect of gray cast iron components

7/30/2019 08 How to Repair Surface Defect of Gray Cast Iron Components

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How to Repair Surface Defect of Gray Cast Iron Components

Using New Method

Muki S. Permana*and Rochim Suratman

**

Mechanical Engineering Department*)

University of Pasundan,Bandung Indonesia, [email protected], Ph. 022- 2019352**)

Bandung Institute of Technology,Bandung - Indonesia, [email protected], Ph. 022-2502265

Abstract

Repair of the gray cast iron component containing surface defect is often necessary in many

industries. From the economic point of view, repair process will decrease production cost at asignificant level. The repair cost is much cheaper compared to the new imported component

which is very expensive. Based on the industrial data, the components to be repaired arenumerous. For example, one of the industries has 52 cylinder heads to be repaired during months

of April and May 2004.

This paper presents an overview a newly developed method on how to repair the surface defect ofgray cast iron components instead of welding. Four methods of repairing have been developed

i.e. Pouring, Powder Filling, Droplet Spray and Turbulence Flow Casting (TFC). TFC method

has been proven to the aim of the present work. The experiment results showed that themetallurgical bonding at the joint was excellent since the fracture location occur at the weld pool

where the tensile strength was about 200 MPa. TFC method, as a new repair solution, is powerfulsince the joint area after repair process is free of cracks and brittleness. The utilization of TFCcan minimize the cooling rate, preventing the formation of the undesirable white cast iron.

Moreover, the homogeneity in properties at the area of repaired was back to the original

characteristics or like new. Through the appropriate selection of parameters, TFC method can beable to repair defects without resulting in the presence of white cast iron, martensite, cracks and

porosities.

Keywords: Repair, gray cast iron, microstructural effect, joint strength, preheating

temperature, pouring time, crack, brittleness,Turbulence Flow Casting (TFC)

1. Introduction

Gray cast iron has been utilized as a structural material in many industries, for example,

automobile components, by virtue of its excellent damping capacity, castability, and

machinability [1]. The components may contain surface defects during casting or under serviceconditions [2]. Casting defects can be in the forms of blowhole, porosity, distortion or

metallurgical defects as segregation, while operational defects are crack, wear, and broken [3]. To

ensure the components still have good performance to be reused in the field, such defects are

usually repaired by fusion welding [4]. Common methods generally applied are arc welding and

oxyacetylene welding using nickel as filler metals [5-7]. However, repairing these materials withsuch conventional welding methods is problematic and difficult especially cracking occurs due to

the brittleness. These processes also require long exposure time at high temperature or high


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pressure, leading to graphite coarsening and high residual stress near joint. Cracking in vicinity of

joint occurs as a result of combination of contraction strain, white cast iron at fusion zone, andhigh carbon martensite in HAZ which is accompanied by micro-cracks (fissures) [4]. While

cracking can, in principle, be avoided by selection of an alternative consumable composition, this

will generally involve a reduction in deposit hardness which may be unacceptable in terms ofservice properties. Where such a material change is inapplicable, the most common preventive

measure is to apply preheat and post-heat performed continually from early till end of process on

the basis that the cooling rate after welding can be reduce with a concomitant reduction in thedifferential contraction strain. But, in this way is impractical because welder must do the repair in

very hot condition. Moreover, it is required muffle furnace for heating the bigger product which

is very expensive besides have to be safe. Furthermore, the latter mechanism of cracking can

normally be overcome by reducing travel speed with attention to arc extinction procedure to

avoid cracking.

From the literatures, therefore, some researchers [8-9] try to repair gray cast iron componentsby other method, among others are diffusion welding by Ni-powder spray welding and solid-state

welding by diffusion bonding performed at elevated temperature and high contact pressure

simultaneously. But, area of joints resulted from these methods is very low strength and

containing porosities. Other development methods, from recent literatures, are ultrasonic insertcasting [10], friction welding [11], and impact-electric current discharge joining [1]. The first

mentioned method is actually still referred to the diffusion bonding, except using ultrasonic wave

instead of temperature and contact pressure to remove oxide layer so that joining can occur. Fromway of same approach, the author also have tried to build another method of droplet spray and

burning-in by pouring molten metal into the surface defect which previously have been given by

oxide-removal like borax but its result was not optimal. The last two methods is not applicable

for joining the bigger components also they are only proper for cylindrical shapes. Thereby, it canbe concluded that instead of using diffusion welding method, it would be better if we remain to

use fusion welding method in repair process to guarantee the nature of mechanical propertiesrequired, but how to do that? In view of the above, it should be developed a new fusion method

to eliminate embrittlement effect, dilution control and lower repair time and even using similar

metal to restore the physical and mechanical properties of the components. Therefore, thecontinuity and originality of materials at area of joint will back to the characteristic prior to the

components undergo defect or failure. Nevertheless, as comparison, this paper conducts all repair

methods mentioned above including new method developed in present work correlating with

metallurgical aspect in consequence of the methods applied.

2. Experimental Details

2.1. Material

The chemical composition of gray cast iron is 3.6 mass%C, 2 mass%Si, 0.6 mass%Mn, 0.06

mass%P, 0.05 masst%S. The microstructure of gray cast iron prior to repair can be seen in fig. 1.


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2.2. Methods

In the experiments, five different methods applied were SMAW (shielded metal arc welding),burning-in, Ni-spray welding, droplet spray and flow casting, are presented. The SMAW method

was performed using specimens prepared in the dimensions of 50 x 15 x 15 mm, at seven

different preheating temperatures of room temperature, 100, 200, 300, 400, 500, 6000C and

9000C for post-heat temperature.

Fig. 1 Microstructure of gray cast iron prior to repair, etched with nital 3%.

The electrode material used was ENiFe-Cl with diameter of 3.2 mm. Single-V grooves was

machined and the butt joints were then continuously welded with SMAW multipass weldingprocess. The experimental welding machine was operated on AC at 100-120 A. The weldingspeed was 0.34 cm/s and interpass temperature was about 600

0C. The second method is burning-

in using molten gray cast iron as filler metal poured into surface defect of specimen which has

dimension 120 x 50 x 25 mm. The specimen was initially preheated by oxyacetylene flame up to

temperature 5000C, then surface defect was covered with borax powder and the powder was

melted by oxyacetylene flame to its melting temperature about 7500C. At this condition, the

surface was filled with molten gray cast iron at pouring temperature about 13500C and

subsequent post-heated at about 9000C. The third method is Ni-spray welding method which was

performed using the same specimens with the SMAW experiment. Preheating and post-heating

temperatures were also same as SMAW welding experiment. The powder material used for this

experiment was standard commercial nickel UTP HA-7 with compositions; 0.75wt%C,3.5wt%Fe, 3.5wt%Si, 7.7wt%Cr, 1.8wt%Br, and the remaining is Ni. The conditions of the

powder and process were spherical particle shape, grain size range of 106 + 20m, melting point

10000C, and neutral flame adjustment. The fourth method is droplet spray method which was

used combinations of SMAW machine and oxyacetylene equipments. The material and size of

electrode used for this experiment was same as observation by SMAW method. The last method

resulted from this work is flow casting method. Joining occurs as a result of convection heattransfer of molten flow into the sand mold which melts the existing base metal inside the mold

and subsequent solidification. Preheating and post-heating continually took place during repair

process. The variations of preheating temperature were in between room temperature till 7000C.

Schematic representation of those five methods of experiment is described in fig. 2.


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Torch

Molten

metal

Stove for Preheat& PWHT

Specimen

Inlet

outlet Sand mold

Powder cup

oxygen & acetylenevalve adjuster

Droplet of molten metalArc

Torch

- +

Electrode FC-20

Specimen

Burning-in Flow Casting

Flame Spray Droplet Spray

Fig. 2. Schematic representation for five methods of experiments.

3. Results and Discussion

Observation resulted from SMAW experiment proved that microstructure at the interface

showed carbide networks as white cast iron along the fusion zone even applying variation ofpreheating temperature. The microstructure can be seen from Fig, 3.

Fig. 3. Microstructure shows carbide networks in the fusion zone resulted from SMAW

experiment.

Fusion zone


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The microstructure resulted from the second experiment of burning-in method using similar

filler metal of gray cast iron is showed by fig. 4a. The mechanism of joining from this experimentis diffusion bonding. Borax powder covered the surface defect prior to filling was aimed to

protect the surface from oxidation when specimen was heated to elevated temperature, but in fact

borax trapped on the surface when solidification of filler metal in weld pool occur. Since thedifficulty of this work, so this experiment can not be applied for repairing surface defect in the

field.

Ni-powder spray method is better than previously experiment method but successful repair

process significantly depend on:

- A proper gas pressure at neutral flame- The distance between torch tip to surface defect- Joining temperature because it is not allowed more than 10000C to avoid fluid flow of

base metal during repair process.

- Duration heat transfer from torch to the base metal- Flame pressure should be controlled to avoid bubble gas which probably trapped in

the interface.

The microstructure from this experiment is showed in fig. 4b. Droplet spray method is very

easy to practice and less time consuming. The disadvantage, there are still porosities about 3 5

% in the interface but lower than porosities resulted from Ni-powder spray about 10 20 %. This

method can be developed to become practical purpose in the field. The microstructure from thisexperiment is showed in fig. 4c. Finally, the excellent method to handle repair process of cast

iron components is flow casting method. The microstructure from this experiment is showed in

fig. 4d. From this picture can be observed that either base metal or weld pool has similar materialand microstructure, so it can be predicted that mechanical properties for both is also the same.

Fig. 4. Microstructure of gray cast iron resulted from different methods.

WeldBase Metal Interface 150 m

Flow Casting (d)

(b) (c)

Burning-In Flame Spray Droplet Spray


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Here can be seen the component of cylinder heads before and after repaired by flow castingmethod.

Fig. 5. Cylinder heads, before and after repair by flow casting method.

Conclusions

The five main points that can be concluded from this work are:

1. SMAW method for repair gray cast iron showed carbide networks at the fusion zone.

2. Burning-in method should be developed in order to be used in the field

3. Flame spray method is better method than SMAW but lower joint strength.

4. Droplet spray can be developed for practical purpose since easy to practice and shorterrepair time consume.

5. Recommendation for repairing gray cast iron components is Turbulence Flow Castingmethod.

Acknowledgements

The authors would like to thank Professor Harsono Wiryosumarto of Bandung Institute ofTechnology on his encouragement and for helpful discussions. Acknowledgement are also made

to forging and casting department of PT. Pindad (Persero), Indonesia, for their permission in

using induction furnace and all equipments used for this work.


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08 how to repair surface defect of gray cast iron components

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