ZIS-Afitteifungen 1986 28 (9) 959-966

Improving the fatigue limit of butt welds in a high tensile bainitic structural steel Heinz Joachim Spies, Klaus Rossler

Mihajlo Starcevic

Mining Academ y, Frieburg

University of Zagreb

Selected from ZISMitfeifungen 1986 28 (9); Reference ZM/86/9/959;Translation 035

To reduce the consumption of rolled steel in future years better use must be made of the advantages of weldable high tensile strength structural steels in light weight and modern, high strength constructions. This trend is impeded by the fatigue strength of welds, which is practically independent of the tensile strength of the parent material. This is because of the notch sensitivity of steels, which increases with strength. While even in the 1960s the notch effect in welded joints was still primarily ascribed to metallurgical notching and internal defects, the realisation is now widespread that senice life is markedly influenced by weld geometry and the condition of the material at the weld toe.

The first approach towards describing the geometrical notch effect in a welded joint goes back to Neuber,' who successfully demonstrated that in addition to material accumulations (projections) a local stress concentration takes place which depends on half the width of the projection and the smallest radius of curvature of the edge. Nowadays the stress concentrations are determined by finite element and edge element methods and approximation solutions, into which various aspects of weld geometry can be entered.

The need to increase the fatigue resistance of welds, together with their static strength was the reason for large scale research. As a result of this, post-weld TIG treatment was internationally recommended as a process for producing a smooth weld Notch overwelding with electrodes is used as a post-weld treatment in the German Democratic Republic?Jo

Hitherto, enchancement of fatigue strength by post- weld TIG after-treatment has been demonstrated on quenched and tempered (Q&T) steels and steels having a ferritic-pearlitic structure.4-1° As yet no results are available concerning the high tensile bainitic structural steel H 75-3, which has a minimum yield strength of 600MPa and is at present being put into production. The objective of the investigation described below was to assess a possible improvement in fatigue resistance of H75-3 using a post-weld treatment.

EXPERIMENTAL PROCEDURE Tables 1 and 2 show the chemical composition and mechanical properties of the 8mm sheet used. The butt joints were made using three passes in a single V groove preparation with a root gap of 1.5mm and a groove angle of 6W on sheets 300mm long held in a clamping jig.

'Garant 85' electrodes were used, and after each pass the welds were air cooled to room temperature. The root was replaced by a capping layer. Faulty samples were eliminated by radiography. Coarse scale was removed from the plates by sand blasting. The post- weld TIG treatment was performed uniformly with an energy of about 8 kJ/cm, starting at the weld toes and then following with the capping layer. Flat samples 50mm in width were prepared from the welded sheets. Fatigue tests were carried out with a stress ratio of - 1 and 0 on Amsler HFP 10 resonance testing machines and with a stress ratio of + 0.2 using a 20 dyne EDZ of

Table 1 Chemical composition of the experimental material, OX,

C Mn Si P S Cr Mo Ni Nb Al Melt analysis 0.085 1.45 0.45 0.011 0.011 1.50 0.35 0.52 0.09 0.039 Materialanalysis 0.081 1.43 0.44 0.010 0.011 1.52 0.35 0.52 0.07 0.054

Table 2 Mechanical properties of the experimental material (longitudinal samples)

%, 1, Rm, 4 7 z, Testing temperature, "C MPa MPa O/O % + 20 0 - 20 - 40 - 60

KCV, J/cm2 at:

810 935 17 67 110 105 55 48 38

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4 Typical course of rupture in TIG-treated samples, X3.

w-

1 Fatigue strength of untreated, TIG-treated and ground butt welds of the experimental material for x = 0 and Po = 90%.

2 for 0 & T steels for x = 0 and Po = 90%.

Comparison of the experimental results with results

6W

HIM'

450

1 3 0 0 60

150

550 600 750 900 N I m * 1050

RPw-

Dependence of fatigue limit on the Rpo, limit for 3 x = 0 and Po = 90% (results H 75-3 = $0.2, Po = 90%).

the Werkstoffpriifmaschen Company Leipzig.

RESULTS

Fatigue strength Each individual value (apart from specimens which did not fracture) was used for a regression calculation, in which the Wohler line specific to the test (Po = 50%) was determined as an angle bisector of the regression straight line N = f(a) and u = f(N). Assuming a scatterband of uD90%: oD5O0/0 = 0.84 and aD10%: uD50% = 1.22 the fatigue limit was calculated for Po =

The regression calculation was performed by the TAKRAF Heavy Mechanical Engineering Combine Company, Leipzig.

Figure 1 shows the Wohler lines for x = 0 and Po = 90% for ground, TIG-treated and untreated butt welds. This comparison shows the improvement in fatigue limit as a result of TIG treatment, even for the high strength structural steel H 75-3 with bainitic structure. Figure 2 shows a comparison with results obtained from Q & T steels. A post-weld TIG treatment shows similar improvements for both groups of steels. This is emphasised in Fig.3. While in the case of rolled material the fatigue limit increases with the yield strength, no connection exists between the increase in fatigue itrength as a result of post-weld TIG treatment and yield strength.

Fracture initiation and propagation In conjunction with systematic hardness measurements, examination of the position of crack initiation showed that as a rule fatigue cracking starts from the weld toe of the final weld pass independently of whether a post-weld treatment was performed or not. Any differences occurring could be ascribed to local notches or other faults. Figure 4 shows examples of the characteristic course of a crack. Starting from the coarse grain heat affected zone of the TIG seam of the capping layer, the crack first runs perpendicularly to the direction of stressing, passing through various structural zones of the weld. In a number of samples, towards the end of the stable crack growth regime the direction of propagation of the crack changed by an a& of about 45" to the

' 90%andP0 = 10%.

304 WELDING INTERNATIONAL 1987 No.4

7 Post-weld TIG treatment with: a) Ideal weld toe geometry; b) Notch effect in weld toe, X50.

main stress by transition to shear rupture. In these cases, the final fracture took place in the same plane. Even microscopic examination of the fracture faces revealed no influence of post-weld TIG treatment on crack propagation. Ductile transcrystalline crack propagation is characteristic of the stable growth of cracks. Intercrystalline crack propagation along coarse grain

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boundaries took place solely in the zone of incipient cracking in the coarse grain zone of a number of samples. As the crack progresses, stress increases at its tip. This leads to the formation of numerous subsidiary crack: which are disposed perpendicular to the fracture surface and parallel with the crack front (Fig.5). The final fracture took place in a ductile manner (Figd).

DISCUSSION OF RESULTS Even in steels with a bainitic structure, the post-weld TIG treatment of butt welds leads to an increase in the fatigue limit. An increase of about 50% took place in the sheet investigated. At the same time the Wohler line exponent increases. The melting of the weld toe eliminates defects and improves the geometry of the weld profile. In this way, the radius of curvature of the weld toe was increased by about a power of ten, from 0.5-3mm up to 20-3Omm. The angle of flank ascent was reduced from 25-30" to 10-25". This corresponds to a reduction in the shape number aK calculated by Sunamoto3 from about 1.8-1.9 down to 1.1-1.2. These results agree with published data.7S8

Scatter of the seam M shape parameters still exists even following the post-weld TIG treatment. This is illustrated by the micrographs of the weld toe shown in Fig.7 and 8. Figure 8 shows the zone of incipient cracking of two samples tested at a stress amplitude of 200MPa ( x = - 1). The number of cycles to fracture in sample 1, which is more than 13 times higher for the same stress, can probably be ascribed to the clearly visible notch at the weld toe of sample 2. The absence of any connection between the fatigue limit and the yield strength is probably also caused by uncontrolled differences in the seam geometry of welded joints subjected to a post-weld TIG treatment.

The notch sharpness changed by the post-weld TIG treatment influences both the stress intensity of defects in the weld zone and also the formation of incipient cracks. However, investigation of the crack path shows that the spread of existing defects played practically no role. In the present case, therefore, the influence of seam geometry on fatigue strength must be due to its influence on the stage of incipient crack formation.

According to Barsom and McNicol" the stress intensity to produce an incipient crack with a given strength and depth of notch is directly proportional to the square root of the notch radius in the range 0.2mm < cp < 7mm. An increase in the notch radius of 0.5mm to Z 7mrn accordingly leads to an increase by a factor of 3.7 in the stress intensity which can be withstood without incipient cracking. A similar relationship is obtained if a start is not made by assuming ideally elastic behaviour of the material in the bottom of the notch, but the effect of elastic restraint is also allowed for. In that case the calculation of the notch effect according to Neuber' or PetersonI2 can be used to assess the influence of the notch radius on the nominal stress which can be withstood without incipient cracks. From this aspect the thesis that the crack initiation phase can be ignored when determining the fatigue strength of welds calls for critical examination. This conclusion was

385

Crack initiation point

Plate surface 1

TIG seam

Direction of crack propagation

Crack initia6bn point

Direction of crack propagation

8 Incipient crack area of TIG-treated samples: a) Weld toe with little notching, number of cycles to fracture = 2.47 * lo6; b) Notch in weld toe, number of cvcles to

(b) fracture = 0.182 lo6, X50.

also drawn from other papers concerning the fatigue behaviour of welded constructional member~. '~J~

The fact that with a comparable geometry, incipient cracking starts from the weld toe of the final pass indicates that structure and internal stress distribution in the zone of the weld toe clearly influence incipient crack formation appreciably. Investigations of stable crack propagation showed that the threshold value for the start of crack growth AKo and the speed of crack growth are influenced by micro~tructure.~~J~ Attention is drawn more particularly to the considerable influence of stressing ( x value) on the AKo value found in the case of coarse grained structure.'6 A metallurgical optimisation of the structure in the heat affected zone should therefore enhance the effect of the post-weld TIG treatment.

SUMMARY Post-weld TIG treatment of butt welds leads to an improvement in fatigue strength even of steels of bainitic structure. In the case of the steel investigated the fatigue limit of post-weld TIG treated butt welds approached

that of ground butt welds and was of the order of magnitude of Q & T steels of equal strength. The improving effect applies similarly to the fatigue limits. As a rule, the incipient crack starts from the coarse grain zone of the TIG seam last welded and at first covers rectilinearly the whole melting limit of such a seam. Final fracture takes place on the opposite side of the sample in the zone of the heat affected area of the TIG seam of the cover layer and of the welding material of the TIG seam. The weld shape improvement and reduction of notch effect achieved by the post-weld TIG treatment should also be achievable right away. At present an attempt is being made to produce advantageous weld geometries by the MIG-MAG welding of butt joints.

References

1

2 3 4 5

Neuber H 'Kerbspannungslehre'. Springer-Verlag Berlin, 1937,

Radaj D: Schweissen undSchneiden 1984 36 57-63. Sunamoto D ef aL Technical Rev, Mitsubishi Ltd, 1979,211-220. Kenyon N era& Bri! U'eldJ 1966 13 (3) 123-137. Asnis Aef aL Nene Hiiffe 1971 16 (11)675-676.

1st edition, 65-68,149.

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6 7 8

Minner H M and SeegerT Sfahlbau 1977 46 (9) 257-262. Minner H M: Dissertation. TH Darmstadt 1981. Musgen B: Stahl u Eken 1983 103 (51 225-230.

12

13

Peterson R E 'Metal fatigue'. New York, 1955,293-306.

Schulze G Schweissen undschneiden 1g82 34 234-238. 9

10 Henkhel K: 'Dissertation'. TH Kail-Max-Stadt, 1983. Neumann K and Niemc K Schweissrechnik Berlin 1984 34 (9)

14 1984 13-21.

Ho N J and Lawrence F V Theoretical and Appl Frac Mech

425-427. 11 Barsom J M and McNicol R C ASTM STP 559,1974,183-204. Rolfe S T and Barsom J M: 'Fracture and fatigue control in structures'. prentice-Hall, 1977,208-231. Grundstoffindustrie, Leipzig 1984,72-93.

l5 16

Tersu lo Ha.?une 19" 67 245-261- Spies H-J, Hiibner P and Pusch G'VEB Deutscher Verlag fiir

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