EVALUATION OF ELECTRON BEAM WELDED AISI 415 STAINLESS STEEL

Sheida Sarafan

Département de génie mécanique École de technologie supérieure

Montréal, Québec, Canada Sheida.Sarafan.1@ens.etsmtl.ca

Priti Wanjara National Research Council Canada

Aerospace, Montréal, Québec, Canada Priti.Wanjara@cnrc-nrc.gc.ca

Henri Champliaud Département de génie mécanique École de technologie supérieure

Montréal, Québec, Canada Henri.Champliaud@etsmtl.ca

Denis Thibault Institut de recherche d’Hydro-Québec (IREQ)

Varennes, Québec, Canada Thibault.Denis@ireq.ca

Louis Mathieu ALSTOM Canada Inc.

Sorel-Tracy, Québec, Canada louis.mathieu@power.alstom.com

ABSTRACT

Sustainable manufacturing for assembly of turbines used in hydro power generation systems is driving the development of advanced technologies targeted to reduce life-cycle costs whilst assuring high performance over the prolonged product life-span. The turbine runner, a critical component in hydro power generation systems, requires weld assembly between the crown, band and blade sub-components. With due consideration of the thick-gauge sections involved, design and fabrication of a turbine runner that would integrate a high energy density technology for assembly, such as vacuum electron beam welding (EBW), has marked potential to achieve deep penetration with a low heat input, thereby rendering a weldment with narrow heat-affected zones (HAZ) and low distortion. In this study, the weldability of thick-gauge section AISI 415 martensitic stainless steels that are widely utilized in hydro turbine manufacturing was investigated by EBW. Particularly, bead-on-plate (BOP) welding of 88 mm-thick AISI 415 plate was carried out using a 42 kW high vacuum EBW system. The characteristics of the weldments, such as fusion zone (FZ) and HAZ microstructures and hardness were evaluated. The microstructural constituents across the weldment for process conditions that rendered near-complete penetration were studied and related to the microhardness evolution.

Key words: Electron beam welding, thick gauge section, wrought AISI 415 martensitic stainless steel, microhardness.

INTRODUCTION

Martensitic stainless steels such as AISI 415 are widely used for hydraulic turbines, valves and pumps in power generation industries [1]. These steels perform well in applications where corrosion and cavitation-erosion resistance, as well as resistance to stress corrosion cracking (SCC), are required. AISI 415 also has the advantage of high strength and good toughness [1]. Due to the size of the turbine components, the development of a high energy density technology, such as EBW, for joining thick gauge sections with a single pass in AISI 415, has many advantages

over multi-pass arc welding technologies, such as autogenous

processing (without filler metal addition), low heat input, and the

capability of minimizing microstructural variation in the

weldment due to single pass welding [3]. In this regard, the

weldability of thick gauge section (thickness up to 88 mm) AISI 415 stainless steel using EBW was studied to understand the

microstructural and microhardness evolution.

EXPERIMENTAL PROCEDURE

The material used was based on ASTM A480 grade AISI 415, a 13% Cr–4% Ni low carbon martensitic stainless steel with a chemical composition of 12.87 wt-%Cr, 3.88 wt-%Ni, 0.72 wt-%Mn, 0.029 wt-%C, 0.56 wt-%Mo, 0.3 wt-%Si, 0.016 wt-%P, 0.0003 wt-%S, 0.036 wt-%N2 and balance Fe measured by AcelorMittal. BOP welding was performed on a coupon machined from an as-received quenched and tempered plate of AISI 415 with dimensions of 200 mm length, 75 mm width and 88 mm thickness (t).

The coupon was demagnetized using a surface demagnetizer (Electro-Matic model A13-1) to achieve a suitable magnetic field. The coupon was then fixed onto the worktable of the EBW system using a clamping fixture. Welding was performed along the length of the coupon using a 42 kW Sciaky W2000 EBW system (60 kV/700 mA) operating with pressure lower than 6.7 × 10-3 Pa. It is noteworthy that preheating prior to welding was performed as an integrated process entirely within the EBW system using a defocused beam that was oscillated in a circular path on the top surface of the AISI 415 coupon along the longitudinal direction [3]. Upon reaching the required pre-heat temperature range (100°C –170°C), EBW was performed.

The weld was then sectioned transverse to the welding direction and prepared metallographically for microscopic examination. Electrolytic etching of the specimens was performed by immersion for 60 seconds in a solution of 10% oxalic acid. An optical microscope (Olympus GX-71) was used to examine the microstructural characteristics. The Vickers microhardness (Struers Duramin A300) across the weld was

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014

November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-39317

1 Copyright © 2014 by ASME and The Crown in Right of Canada

measured near the top, mid-thickness and root using a load of 500 g and an indent interval of 0.3 mm.

RESULTS AND DISCUSSION Microscopic Examination

The typical evolution in the microstructure across electron beam welded (EB welded) AISI 415 is illustrated in Fig. 1. The microstructure of the base metal (BM) of AISI 415 consists predominantly of martensite laths, about 5% delta ferrite and 10% retained austenite depending on the heat treatment (Fig. 1a). This is consistent with BM microstructures reported in [4, 5]. The FZ microstructure of the welds consisted of columnar grains with a dendritic structure within which a network of delta ferrite was present (Fig. 1b). On either side of the FZ, different HAZs were apparent, as illustrated in Fig. 1c. The HAZ just adjacent to the FZ (HAZ1), often referred to as the partially melted zone (PMZ), was difficult to distinguish in EB welded AISI 415. The remaining four distinct and distinguishable HAZs in the weld, namely HAZ 2-5 (Fig 1c and d), have microstructures that can be interpreted through the ternary Fe-Cr-Ni phase diagram [5]. This microstructural evolution observed in EB welded AISI 415 is relatively similar to reported work on EBW of thick gauge section (60 mm) CA6NM [3], which is a 13% Cr-4% Ni martensitic stainless steel in cast form; a difference in the relative size of the FZ and HAZs was noted, which may be attributed to the different heat input applied to penetrate the required gauge thickness.

Fig. 1. Typical evolution in the microstructure of AISI 415 electron

beam weld: (a) BM, (b) FZ, (c) HAZ2-4 and d) HAZ5.

Microhardness Fig. 2 reveals the typical evolution in the microhardness

profile at the mid-thickness of the AISI 415 electron beam weld. The hardness of the BM was 296 ± 3 HV. A minimum hardness of 291 ± 18 HV occurred in HAZ5 and the local softening is due to tempering of the martensite structure of the BM that was in the quenched and tempered condition. A maximum value of 396 ± 12 HV occurred in HAZ4, relatively close to the HAZ3/HAZ4 boundary due to the formation of virgin martensite as a result of heating well above Ac1. Within HAZ3 and HAZ2 there was a progressive decrease in the hardness, such that at the FZ boundary a second hardness maximum of 373 ± 9 HV was apparent. This hardness evolution is similar to results reported for multi-pass flux-cored arc-welded 13% Cr-4% Ni martensitic

steels with a gauge thickness of 4 mm [4] as well as EB welded CA6NM that had a thickness of 60 mm [3].

200

250

300

350

400

450

0 10 20 30

Har

dnes

s (H

V)

Distance from weld centerline (mm)

Middle

FZ HAZ4 HAZ5 BM

HAZ2 HAZ3

Fig. 2. Typical hardness evolution from the BM to the FZ at mid-

thickness of EB welded AISI 415.

CONCLUSIONS

Based on the results of the present research the following conclusions can be drawn: Electron beam welds of AISI 415 are composed of 4 HAZs

and each HAZ is differentiable by specific microstructural characteristics that correspond to particular microhardness values.

The typical hardness profiles exhibited a hardness maximum (396 ± 12 HV) and minimum (291 ± 18) in HAZ4 and HAZ5, respectively. The average hardness in the fusion zone was 374 ± 16 HV.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from Alstom Hydro, Hydro-Québec, National Research Council of Canada (NRC) and the Natural Sciences and Engineering Research Council of Canada (NSERC) for this project, which is a part of CReFARRE-Consortium de recherche en fabrication et réparation des roues d’eau. The authors also would like to thank X. Pelletier of NRC for his technical support related to the welding trials and metallographic preparation.

REFERENCES

1. Gooch, T.G., 1995, “Heat treatment of welded 13%Cr–4%Ni martensitic stainless steels for sour service,” Welding Journal, Vol. 74(8), pp. 213-223.

2. Schultz, H., 1993, “Electron beam welding,” Cambridge, England, Abington, pp. 1-5.

3. Sarafan, S. et al., 2013, “Characteristics of electron beam welded CA6NM,” proceedings, Material Science and Technology, Canada, pp. 720-732.

4. Thibault D. et al., 2009, “Residual stress and microstructure in welds of 13% Cr–4% Ni martensitic stainless steel,” Journal of Materials Processing Technology, Vol. 209(4), pp. 2195-2222.

5. Godin, S. et al., 2014, “An experimental comparison of weld-induced residual stresses using different stainless steel filler metals commonly used for hydraulic turbines manufacturing and repair”, Materials Science Forum Vols. 768-769, pp 628-635.

6. Folkhard, E., 1988, “Welding metallurgy of stainless steels”, New York, Springer-Verlag Wien, pp. 179-183.

2 Copyright © 2014 by ASME and The Crown in Right of Canada