Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (2016)

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PAPER REF: 6295

RESIDUAL STRESS RELIEF IN WELDED JOINT WITH

LOW MARTENSITIC TRANSFORMATION

TEMPERATURE WIRE F.J.S. Oliveira

1(*), H.F.G. Abreu

1, C.C. Silva

1, M. Cindra Fonseca

2

1Department of Metallurgical and Materials Engineering, Federal University of Ceará-UFC, Fortaleza, Brazil 2Department of Mechanical Engineering, Federal Fluminense University - UFF, Niteroi, RJ, Brazil (*)Email: fjoliveira7@gmail.com

ABSTRACT

This work presents a comparative study of the levels of residual stresses in two welded joints, produced with two different filler metals welded by Flux Cored Arc Welding (FCAW). the first one was martensitic stainless steel wire Fe-12,5% Cr-4% Ni with low transformation temperature (Ms~180 oC) and the second one a conventional low alloy steel Fe -1.25% Cr-0.5% Mo. The base metal was a 25 mm thick quenched and tempered AISI 4140 . The welding process was performed in a workbench robot with multiprocess source. Residual stresses were measured on both surface of welded joints (top and root) using X-ray diffraction method. Microstructure and hardness in weld metal, heat affected zone and base metal were evaluated. The results showed that the joint produced with the low temperature martensitic transformation presented significant levels of compressive residual stress, unlike the other joint which presented tensile residual stress at most of the measured points. The microstructure presented predominance of bainite and martensite. The hardness was influenced by the thermal cycle of welding, but had no direct relationship with the residual stresses generated on the surfaces of the joints.

Keywords: Multi-pass welding, cored wire, martensitic transformation, residual stress, microstructure, hardness.

INTRODUCTION

Due to the large heat input concentrated in a small area during the welding process there is the occurrence of different expansion levels in the weld region during heating. The same occurred in contraction during cooling. Dilation of the piece that experience high temperatures is hampered by the adjacent region experiencing much lower temperatures. The result is the emergence of residual stresses. Control measures to minimize these residual stresses should be adopted since they can be decisive for a structure to fail or not. One way to reduce or eliminate residual stresses in welded joints is a post weld heat treatment (PWHT). But not always the PWHT is a viable process. It depends on the size and complexity of the pieces.

It is understood and consolidated in the literature that phase transformations arising from thermal processes are metallurgical phenomenon which causes changes in the state of residual stresses in metallic materials. A phase transformations extensively studied and researched is the martensite transformation at a low temperature. Several researchers have published papers to prove that this may promote a decrease in residual stresses caused by the welding, thus eliminating the PWHT.

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According Bhadeshia (2004), the martensitic transformation is accompanied by deformation and dilation. Recent studies of Thibalt et al (2010), Tavares (2013) and Nunes (2015) have shown that the transformation phase in steels, specifically martensitic occurring at temperatures around 180° C can cause a decrease in residual tensile stresses arising in the weld metal due to deformation occurred consequence of the displacive tranformation. At higher temperatures, the martensitic transformation has its effect in reducing residual stresses greatly compromised because after the expansion, the martensite transformed continues to contract, and since it has a high yield strength, residual stresses due to contraction may again rise considerable (Alberry & Jones, 1977). The present work studied the level of residual stresses in welded joints of 25 x 150 x 200 mm quenched and tempered AISI 4140 steel using as filler metal cored wire AWS 5:22 E410NiMoT1-1 / 4 which undergoes martensitic transformation at low temperature and cored wire AWS E81T1 B2. A comparison of the levels of residual stresses in welded joints measured by X-ray diffraction of was raised to assess the effect of the martensitic transformation at low temperature in them. Residual stress, hardness and microstructure were evaluated in the base metal, heat affected zone and weld metal at top and root of the joints. EXPERIMENTAL PROCEDURES

Materials

The base metal used in this work was AISI 4140 steel quenched at 860 ° C for 1 hour, cooled in oil and tempered at 480 ° C for 1 hour followed by air-cooling. The filler metals used for making joints were martensitic stainless steel 12.5 Cr 4.0 Ni and low alloy steel 1.25 Cr 0.5 Mo. The chemical compositions of the base metal and filler metals are shown in Table 1.

Table 1 - Chemical compositions of the base metal and the filler metal (in wt%). Material Fe C Si Mn Cr Ni Mo S P

Base (4140) Balance 0,39 0,24 0,82 0,83 - 0,22 0,006 0,010

Filler (CrNi) Balance 0,04 0,7 1,2 12,5 4,0 0,40 - -

Filler (CrMo) Balance 0,08 0,8 1,25 1,25 - 0,5 0,03 0,03

Welding Procedure

The welding of the two joints was performed by tubular wire process (FCAW) through a robotized bench with multiprocess source and MIG/MAG torch welding as show in Fig. 1a. It has been used a fixation system by mechanical clamps as shown in Fig. 1b. Preheating and interpass temperature was 215 ºC (±15), based on standard PETROBRAS N-133N WELDING (2013).

Fig. 1 - a) Robot Kuka b) constraint system.

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The joint made the low martensitic transformation temperature filler was named only martensitic and the other produced with filler metal of low alloy steel was named Low alloy. Table 2 show welding parameters.

Table 2 - Welding parameters.

Joint Ø (mm) Gas U

(V) I

(A) V

(mm/s) Valim

(m/min) Heat imput

(kJ/mm)

Martensitic 1.6 Self-protected 28.8 200 5 5.6 1.15

Low alloy 1.2 75%Ar+25%CO2 27 180 4.7 7 1.04

The martensitic joint received 14 pass and the low alloy joint 19 pass. This happened because of different wire diameter and deposition rates. Residual stress measurements

The residual stress were measured by X-ray diffraction. It is a technique widely used because preserve the integrity of the joint and classified as non-destructive (Olabi, 2012). The equipment used was the X-stress 3000 Stress Analyzer by sen2ψ method, radiation Cr Kα (λ = 2,29092 Ǻ) diffracting at plain {2 1 1}, 2θ incidence angle of 156,8º and inclination angles (ψ) 0º, 18º, 27º, 33º e 45º by 10 s. The residual stress was measured in transverse and longitudinal directions to weld bead. Measurements were made at points in the center of each weld bead, at HAZ on both sides and base metal.

Microstructural characterization and microhardness profiles

Metallographic preparation consisted of sanding, polishing and etching with Nital 2% to the metal base of both joints and weld metal of low alloy joint. The etching of the martensitic weld metal was made with 10%-Chromic acid. The base metal microstructure, HAZ and weld metal were observed by optical microscope. For microhardness profiles was used a Shimadzu micro-hardener and included regions of the base metal, the HAZ and the weld metal. Vickers scale and a load of 100 g for low alloy joint and 200 g for martensitic were used. Readings were taken every 0.2 mm.

RESULTS AND CONCLUSIONS

Residual stress

The results from residual stress on the top surface of the welded joints are shown in Figure 2. The weld metal of the joint produced with the martensitic stainless steel wire Fe-12% Cr-5% Ni if presented entirely compressive in the transverse and longitudinal directions. The compressive residual stresses in the weld metal martensitic low temperatures reached values of up to 310 MPa in the transverse direction (Figure 2a) and up to 180 MPa in the longitudinal direction (Figure 2b). The weld metal produced with low alloy steel presented compressive in transverse direction, but one of the points presented trative, while in the longitudinal direction entirely tensile with values between 150 Mpa e 360 Mpa.

The heat affected zone (HAZ) of the martensitic joint was similar to the weld metal , with compressive residual stress (except a point) in the joint made of martensitic steel and tensile residual stress in welded joint with low alloy steel.

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In both joints, the base metal presented tensioned, because the heat treatment subjected.

These results, at HAZ and weld metal, were similar to results obtained by other researchers, for example Thibault et al (2010) and Kannengiesser et al (2009) showed that low temperature martensitic weld metal leads to a reduction of residual stresses or even generating compressive residual stress in the fusion and heat affected zones.

Fig. 2 - Residual stresses on the top surface of joints, a) transverse, b) longitudinal.

The Figure 3 shows the results of residual stress at the root of the joints. In the transverse direction (Fig. 3a) both joints presented tensile residual stress in the weld and HAZ metal. For martensitic joint, this result differs from stress state found on top of the joint. However in the longitudinal direction compressive residual stresses were found in the weld metal of both joints, -110 MPa in martensitic and -70 Mpa in low alloy (Fig 3b). On the other hand, in the HAZ region, residual stress presented completely tensile in both cases.

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Fig. 3 - Residual stresses on the root surface of joints, a) transverse, b) longitudinal.

Microstructural characterization and microhardness profiles

The base metal presented tempered martensite according heat treatment previously submitted before welding. Figure 4 show microstructures at optical microscopy of the weld metal and HAZ of both joints. The martensitic weld metal joint presented predominantly martensite (Figure 4a and Figure 4b). In HAZ (figure 4c) can be observed the presence of coarse martensite and upper bainite. In figure 4d it can be observed grain growth. Both images presented of HAZ coarse grain.

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Fig. 4 - Martensitic joint a) weld metal top b) weld metal root c) HAZ top d) HAZ root 50x; Low alloy joint e) weld metal top f) weld metal root g) HAZ top h) HAZ root; optical microscopy 500x.

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The low alloy joint presented acicular ferrite and primary ferrite at weld metal (Figures 4e and 4f). The HAZ coarse grain also presented martensitic microstructure (Figures 4g and 4h), result similar to martensitic joint influenced by the same base metal and practically the same energy used.

Martensite formation in most of the coarse grain HAZ was due to high hardenability of the AISI 4140 steel and the high cooling rates due to the elevated relative thickness of the plate.

Figure 5 show microhardness profiles in low alloy and martensitic joints from the weld metal to the base metal.

The base metal has reached values between 390-425 HV, and confirms the results of the quenching and tempering performed in 4140 AISI steel. In HAZ both joints presented values microhardness majority in the range 375-550 HV, although differences in values, these results are similar to Ramkumar et al (2014). The HAZ of the martensitic joint was influenced by the thermal cycle of welding, and had a reduction in root.

Fig. 5 - Microhardness profile

In weld metal of the martensitic joint microhardness range was between approximately 450-490 HV, values different from reported by others authors. Thibault et al (2009) found microhardness in range 360-380 HV approximately, using same welding process and heat input, with different base metal.

The weld metal of the low alloy joint presented lower values than martensitic joint, between 250-350 HV, with root joint reaching level medium 330 HV while top joint 260 HV. This difference between top and root occurred because the diffusion process is greater at the root.

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Based on the results of the present work can be concluded:

- The longitudinal residual stress at martensitic joint underwent strong influence of martensitic transformation at low temperature and if presented compressive in the weld metal.

- The low alloy joint presented tensile residual stress in the majority of measured points.

- The microstructure of martensitic joint presented predominance of tempered martensite.

- The microhardness reached high values at martensitic joint, mainly in HAZ region, leading to greater crack susceptibility in this zone.

ACKNOWLEDGMENTS

The authors thank CAPES / FUNCAP for financial support to the design and LACAM and ENGESOLDA laboratories of the Federal University of Ceará (UFC) and stress analysis laboratory of Fluminense Federal University UFF.

REFERENCES

[1]-Bhadeshia, H. K. D. H., “Developments in Martensitic and Bainitic Steels: Role of the Shape Deformation”, Mat. Sci. Eng. A, 378, 34-39, 2004.

[2]-Jones, W. K. C. and Alberry, P. J.: ‘A model for stress accumulation in steels during welding’, Proc. Conf. on ‘Residual stresses in welded construction and their effects’, London, UK, November 1977, TWI, Vol. 1, 15–26.

[3]-Kannengiesse, T, Kromm, A. Formation of welding residual stresses in low transformation temperature (LTT) materials. São Paulo: Welding Inspection, Vol. 14, No. 1, p.074-081, 2009.

[4]-Nunes, C. S., Dissimilar Joints: use the maraging 350 and 250 as filler material in welds. (Thesis). Metallurgical and Materials Engineering, Federal University of Ceará, Fortaleza, 2015.

[5]-Olabi, A. G.,Rossini, N.S,Dassisti, M. Benyounis, K.Y. Methods of measuring residual stresses in components. Materials and Design 35, 572–588. 2012.

[6]-Ramkumar, K.D., Reddy, M. P., Willian A. A. S., Prashant, M. M., Kumar, S.N.S., Arivazhagan, N., Narayanan, S., Assessment of Mechanical Properties of AISI 4140 and AISI 316 Dissimilar Weldments. Procedia Engineering 75, p. 29 – 33. 2014.

[7]-Tavares, W. S. Influence of martensitic transformation at low temperature in residual stress level and crystallographic texture of Welded Joints of the steel used in the Petroleum Industry. (Thesis). Metallurgical and Materials Engineering, Federal University of Ceará, Fortaleza, 2013.

[8]-Thibault, D., Bocher, P., Thomas, M. Residual stress and microstructure in welds of 13%Cr–4%Ni martensitic stainless steel. Journal of Materials Processing Technology 209, p.2195-2202. 2009.

[9]-Thibault, D., Bocher, P., Thomas, M., Gharghouri, M., Côté, M. Residual stress characterization in low transformation temperature 13%Cr-4%Ni stainless steel weld by neutron diffraction and contour method. Materials Science and Engineering A 527, p.6205-6210. 2010.