Solidification cracking in austenitic stainless steel welds

Solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as S, P and alloy elements such as Ti, Nb. The WRC-92 diagram can be used as a general guide to maintain a desirable solidification mode during welding. Nitrogen has complex effects on weld-metal microstructure and cracking. In stabilized stainless steels, Ti and Nb react with S, N and C to form low-melting eutectics. Nitrogen picked up during welding significantly enhances cracking, which is reduced by minimizing the ratio of Ti or Nb to that of C and N present. The metallurgical propensity to solidification cracking is determined by elemental segregation, which manifests itself as a brittleness temperature range or BTR, that can be determined using the varestraint test. Total crack length (TCL), used extensively in hot cracking assessment, exhibits greater variability due to extraneous factors as compared to BTR. In austenitic stainless steels, segregation plays an overwhelming role in determining cracking susceptibility.     

奥氏体不锈钢焊缝超声回波信号的匹配追踪处理

研究了奥氏体不锈钢焊缝组织的金相显微结构及其对超声检测的影响,利用超声相控阵检测技术对定制的奥氏体不锈钢对接焊接接头对比试块中不同深度(10、30、50、70 mm)、φ2 mm×30 mm的横通孔缺陷进行了不同波型(横波和纵波)的检测,采用匹配追踪后处理方法对超声回波信号进行了处理。结果显示:奥氏体不锈钢焊缝组织结构复杂,晶粒粗大,各向异性明显,对超声检测产生严重的声能衰减,纵波检测奥氏体不锈钢焊缝中较深缺陷(50 mm)的能力强于横波检测,且匹配追踪对奥氏体不锈钢焊缝超声检测回波信号的处理不仅能有效抑制噪声信号、提高信噪比,还能提取出被淹没在噪声信号中的缺陷信号。     

Embrittlement of austenitic stainless steel welds

To prevent hot-cracking, austenitic stainless steel welds generally contain a small percent of delta ferrite. Although ferrite has been found to effectively prevent hot-cracking, it can lead to embrittlement of welds when exposed to elevated temperatures. The aging behavior of type-308 stainless steel weld has been examined over a range of temperatures 400–850C for times up to 10,000 hr. Upon aging, and depending on the temperature range, the unstable ferrite may undergo a variety of solid state transformations. These phase changes affect creep-rupture and Charpy impact properties.     

Creep crack growth simulations in 316H stainless steel

Virtual methods of predicting creep crack growth (CCG), using finite element analysis (FE), are implemented in a compact tension specimen, C(T). The material examined is an austenitic type 316H stainless steel at 550 °C, which exhibits power-law creep–ductile behaviour. A local damage-based approach is used to predict crack propagation and the CCG rate data are correlated with the C^∗ parameter. Two-dimensional elastic–plastic–creep analyses are performed under plane stress and plane strain conditions. Finite element CCG rate predictions are compared to experimental data and to the NSW and modified NSW (NSW–MOD) CCG models’ solutions, which are based on ductility exhaustion arguments. An alternative version of the NSW–MOD model is presented for direct comparison with the FE implementation. The FE predictions are found to be in agreement with the appropriate analytical solutions, and follow the trends of the experimental data at high C^∗ values. Accelerated cracking behaviour is observed experimentally at low C^∗ values, which is consistent with the standard plane strain NSW–MOD prediction. The FE model may be developed to predict this accelerated cracking at low C^∗ values so that the trends between CCG rates at high and low C^∗ values may be determined.     

Creep behaviour leading to Type IV cracking for service-exposed 1.25Cr-0.5Mo steel welds

To ensure reliability of elevated temperature components, the creep behaviour of weldment must be predicted since the ultimate failures mostly take place at this tiny region. In the case of low alloy ferritic steels, the most likely failure mode of equipment operated for long hours should be Type IV cracking, which is defined as preferential damage evolution at the Intercritical HAZ (ICZ). Despite the importance of this phenomenon, there have been some uncertainties remained unsolved. In order to elucidate the cause and accelerating factors of Type IV cracking, creep behaviours of cross-weld and the ICZ microstructure have been examined in the present work using service-exposed 1.25Cr-0.5Mo steel.Onset time to Type IV failure significantly reduced when tested by spirally notched cross-weld specimens as a result of concentrated damage accumulation at the root of a vee notch, revealing that multiaxaial stress state could play a key role in Type IV failure.The feature of creep damage suggests that grain boundary damage leading to Type IV cracking is caused by the sliding of grain boundaries around fine grains which are considered to be the products of partial transformation during welding. Heterogeneous damage evolution to the level of facet cracking surrounded by damage free grains raises the fundamental question on the validity of a generally accepted assumption, namely, that stress of grains associated with a grain boundary cavity will be off-loaded. As a matter of fact, a clear evidence that grain boundary cavitation accelerates the strain rate at the tertiary regime has not been observed in creep curves of simulated ICZ specimens, owning a bimodal microstructure expected at the ICZ in whole gauge length.Difference in the susceptibility to Type IV cracking has been found in materials with the same alloying elements and the vulnerability of the ICZ microstructure is not necessarily dependent upon creep strength of parent material.Considerable metallurgical factors to shorten the onset time to Type IV damage and the effectiveness of strain rate measurement as a potential technique for the life assessment shall be discussed.     

Creep characterization of a type 316 austenitic steel

Creep data on a type 316 austeni tic steel are presented covering the temperature range 550-675°C and in volving test times up to 30 000 h. The data have been used to inv estigate the efficiency of traditional and recent methods of presenting creep data , the formulation of creep constit utive equations, and the valid ity of certain parametric relationships designed to aid extrapolation. Th e article introduces a form of the steel which will subsequently appear in a study of multi-axial stress/creep relationships and the development of a design method for creep conditions.     

不锈钢母材及其焊缝金属材料单拉本构关系研究

对12个奥氏体型及12个双相型不锈钢正面角焊缝和侧面角焊缝连接试件进行了单调拉伸试验,考察了不同焊接工艺对角焊缝连接力学性能的影响。结果表明：采用氩弧焊焊接工艺的不锈钢角焊缝试件破坏面与电弧焊焊接工艺的试件破坏面形状相差较大,后者破坏面更加光滑;同时由于受到复杂应力的作用,正面角焊缝试件的真实破坏角度并不为相关规范规定的理论值45°;对于奥氏体型不锈钢角焊缝,氩弧焊试件与电弧焊试件的强度比分别为1.03 (正面角焊缝试件)及1.13 (侧面角焊缝试件),相对变形量之比为1.46及1.11;而对于双相型不锈钢角焊缝,两者的强度比分别为1.12和1.04,相对变形量之比为1.66及1.45;氩弧焊试件表现出了更好的力学性能。对于两种不锈钢材料,正面角焊缝强度均远大于侧面角焊缝的强度,建议在工程设计和相关规范的编制/修订中考虑正面角焊缝强度提高的影响。     

Crack resistance of austenitic stainless steel pipe and pipe welds with a circumferential crack under monotonic loading

This paper describes the experimental studies carried out on cracked austenitic stainless steel pipe and pipe welds under bending loads. Pipe welds were produced by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW). Fracture resistance curves for pipe and pipe welds were compared. Results indicate that the fracture resistance of pipe and pipe weld (GTAW) is comparable but that of pipe weld (GTAW+SMAW) is inferior. Cracks do not deviate from their original plane during propagation as observed in the cases of carbon steel pipe and pipe welds. The fracture resistance of pipe welds does not depend on the loading histories to which it has been subjected prior to fracture test. Initiation and crack propagation were observed prior to the maximum moment. An existing limit load expression is applicable for the pipe base material but gives non‐conservative results for the pipe welds. Multiplication factors have been suggested for the pipe welds for evaluation of limit loads using the existing expression. Fracture resistance for the pipe and compact tension specimens have also been compared for base material and welds.     

Laser power coupling efficiency in conduction and keyhole welding of austenitic stainless steel

Laser welding of thin sheets of AISI 304 stainless steel was carried out with high power CW CO₂ laser. The laser power utilized in the welding process was estimated using the experimental results and the dimensionless parameter model for laser welding; and also the energy balance equation model. Variation of laser welding efficiency with welding speed and mode of welding was studied. Welding efficiency was high for high-speed conduction welding of thin sheets and also in keyhole welding process at high laser powers. Effect of pre-oxidization of the surface and powder as filler material on laser power coupling is also reported. The paper also discusses effect of microstructure on the cracking susceptibility of laser welds.     

Chi-phase precipitation in a duplex stainless steel

The aim of this study is to investigate the precipitation of intermetallic phases, especially the chi-phase, in a 45N (type UNS S31803) duplex stainless steel through aging heat-treatments carried out at 700 and 750 °C. Two intermetallic phases are detected: chi (χ) and sigma (σ). The χ-phase precipitates at ferrite/ferrite grain boundaries prior to the σ-phase precipitation, which occurs preferentially at ferrite/austenite interfaces and at ferrite/ferrite grain boundaries. The σ-phase precipitation is a eutectoid type reaction of ferrite leading to σ-phase phase and austenite. The χ-phase is consumed in the σ-phase precipitation after becoming completely surrounded by both the σ-phase and the newly formed austenite.     

Improved creep-resistance of austenitic stainless steel for compact gas turbine recuperators

Abstract

Primary surface recuperators (PSRs) are compact heat-exchangers made from thin-foil type 347 austenitic stainless steel, which boost the efficiency of land-based gas turbine engines. Compact recuperators are also an essential technology for some new microturbines. Solar turbines uses foil folded into a unique corrugated pattern to maximize the primary surface area for efficient heat transfer between hot exhaust gas on one side, and the compressor discharge air on the other side of the foil. Allegheny-Ludlum produces 0.003–0.004 inches thick foil for a range of current turbine engines using PSRs that operate up to 660°C. One goal of this team-effort project is to modify the processing to enable improved creep resistance of such 347 stainless steel foils at 650–700°C. Laboratory-scale processing modification experiments recently have demonstrated that dramatic improvements can be achieved in the creep resistance of such typical 347 stainless steel foils. The modified processing enables fine NbC carbide precipitates to develop during creep at 650–700°C, which provides strength even with a fine grain size. Such improved creep-resistance allows greater flexibility in optimizing the cost-performance relationship as increased demands are placed on the PSR at higher operating temperatures. The next challenges are to better understand the nature of the improved creep resistance in these 347 stainless steel foil, and to achieve similar improvements with scale-up to commercial foil production.     

Fatigue crack growth of a metastable austenitic stainless steel

The fatigue crack growth behavior of an austenitic metastable stainless steel AISI 301LN in the Paris region is investigated in this work. The fatigue crack growth rate curves are evaluated in terms of different parameters such as the range of stress intensity factor ΔK, the effective stress intensity factor ΔK_eff, and the two driving force parameter proposed by Kujawski K¹.The finite element method is used to calculate the stress intensity factor of the specimens used in this investigation. The new stress intensity factor solution has been proved to be an alternative to explain contradictory results found in the literature.Fatigue crack propagation tests have been carried out on thin sheets with two different microstructural conditions and different load ratios. The influence of microstructural and mechanical variables has been analyzed using different mechanisms proposed in the literature. The influence of the compressive residual stress induced by the martensitic transformation is determined by using a model based on the proposal of McMeeking et al. The analyses demonstrate the necessity of including K_max as a true driving force for the fatigue crack growth. A combined parameter is proposed to explain the effects of different variables on the fatigue crack growth rate curves. It is found that along with residual stresses, the microcracks and microvoids are other factor affecting the fatigue crack growth rate in the steel studied.     

Fatigue failures of austenitic stainless steel orthopedic fixation devices

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.     

316L奥氏体不锈钢中时效条件下析出相演变行为的研究

通过定量金相,SEM&EDS、TEM等实验技术分析316L奥氏体不锈钢中析出相随时效时间、温度的变化,并测定析出相的体积分数与尺寸.结合热力学计算表明:在316L奥氏体不锈钢中,经850℃时效处理后,析出相为M23C6型碳化物,且随着时效时间的延长,析出量明显增多,尺寸增大;经650℃时效处理100 h后,主要析出相类型为χ相.     

Microstructural effects on the short crack behaviour of a stainless steel weld metal during low-cycle fatigue

An experimental study into microstructural effects on short fatigue crack behaviour of 19 stainless steel weld metal smooth specimens during low-cycle fatigue is performed by a so-called ‘effective short fatigue crack criterion’. This material has a mixed microstructure in which it is difficult to distinguish the grains and measure the grain diameter. The columnar grain structure is made up of matrix-rich δ ferrite bands, and the distance between the neighbouring rich δ ferrite bands is an appropriate measurement for characterizing this structure. Particularly, the effective short fatigue cracks (ESFCs) always initiate from the bands of δ ferrite in the matrix in the weakest zone on one of the specimen surface zones which is orientated in accordance with the inner or outer surface of welded pipe from which the specimens were machined. These cracks exhibit characteristics of the microstructural short crack (MSC) and the physically small crack (PSC) stages. The average length of the ESFCs at the transition between MSC and PSC behaviour is ≈40 μm, while the corresponding fatigue life fraction is ≈0.3 at this transition. Different from previous test observations, the growth rate of the dominant effective short fatigue crack in the MSC stage still shows a decrease with fatigue cycling under the present low-cycle fatigue loading levels. A statistical evolution analysis of the growth rates reveals that the short fatigue crack growth is a damage process that gradually evolves from a non-ordered (chaotic) to a perfectly independent stochastic process, and then to an ordered (history-dependent) stochastic state. Correspondingly, the microstructural effects gradually evolve from a weak effect to a strong one in the MSC stage, which maximizes at the transition point. In the PSC stage, the effects gradually evolve from a strong to weak state. This improves our understanding that the short crack behaviour in the PSC stage is mainly related to the loading levels rather than microstructural effects.     

Creep failure model of a tempered martensitic stainless steel integrating multiple deformation and damage mechanisms

A new model considering both deformation and damage evolution under multiple viscoplastic mechanisms is used to represent high temperature creep deformation and damage of a martensitic stainless steel in a wide range of load levels. First, an experimental database is built to characterise both creep flow and damage behaviour using tests on various kinds of specimens. The parameters of the model are fitted to the results and to literature data for long term creep exposure. An attempt is made to use the model to predict creep time to failure up to 10⁵ h.     

Powder injection moulding of premixed ferritic and austenitic stainless steel powders

In this experimental work, powder injection moulding (PIM) of premixed 316L and 430L gas-atomized powders was developed to obtain duplex stainless steels. A multicomponent binder constituted of high density polyethylene (HDPE) and paraffin wax (PW) in a volume ratio 50/50 was selected for the process. Feedstocks with powder loadings of 50, 65, 68 and 70 vol.% were prepared. Mixing experiments were carried out at 170 °C according to differential scanning calorimetry (DSC) results. The rheological characterization of feedstocks allowed establishing different rheological parameters as power flow index (n) and activation energy (E_a) in order to know their suitability for injection moulding. Critical powder volume concentration (CPVC) was determined by means of oil absorption method and a rheological model. The feedstock was injected at 170 °C and three-point bending and tensile parts were obtained. Thermogravimetrical analysis (TGA) of binder allowed us to design the thermal debinding cycle and sintering was carried out in low vacuum at different temperatures. Finally, mechanical properties such as hardness and tensile strength were evaluated.     

A study on low magnetic permeability gas tungsten arc weldment of AISI 316LN stainless steel for application in electron accelerator

Low magnetic permeability is an important criterion in selection of the material of construction of beam pipes and vacuum chambers of electron accelerators for safeguarding against distortion of the magnetic field. In the modified design of new 20 MeV/30 mA Injector Microtron for the existing synchrotron radiation sources Indus-1 and Indus-2, AISI 316 LN stainless steel has been identified as the material of construction of its vacuum chamber. Welding of AISI 316LN stainless steel with conventional filler alloys like ER316L and ER317L of AWS A5.9 produces duplex weld metal with 3–8% ferro-magnetic delta ferrite to avoid solidification cracking. The results of the study has demonstrated that GTAW of AISI 316LN SS with high Mn adaptation of W 18 16 5 N L filler produced a crack free non-magnetic weld with acceptable mechanical properties. Moreover, AISI 316LN stainless steel is not required to be solution annealed after the final forming operation for obtaining a low magnetic permeability, thereby avoiding solution annealing of large vacuum chamber in vacuum/controlled atmosphere furnace and associated problems of distortion. Besides Injector Microtron, the study also provides useful input for design of future indigenous accelerators with vacuum chambers of austenitic stainless steel.     

一种新型核级奥氏体不锈钢焊接接头的晶间腐蚀研究

利用"不锈钢硫酸-硫酸铜腐蚀实验"、光学显微镜和扫描电镜等测试方法分别对未敏化和经敏化处理(650℃×100 h)的800H和新型Cr18Ni30Mo2Al3Nb合金焊接接头的抗晶间腐蚀性能进行研究,结果表明,800H和Cr18Ni30Mo2Al3Nb接头焊缝组织均为单一的奥氏体基体,800H合金中TiN缺陷处易引起点蚀,而Cr18Ni30Mo2Al3Nb无明显点蚀现象;对比腐蚀失重、腐蚀深度等实验结果,未敏化和敏化态Cr18Ni30Mo2Al3Nb焊接接头的抗晶间腐蚀性能明显优于相同状态的800H。     

Delayed cracking of low‐nickel austenitic stainless steel studied with constant load tensile testing

Delayed cracking in unstable low‐Ni austenitic stainless steel 204Cu was studied by constant load tensile testing. The developed testing arrangement enabled a systematical examination on the effect of applied stress, strain‐induced α′‐martensite and internal hydrogen content on time to fracture. Volume fraction of strain‐induced α′‐martensite was shown to affect cracking kinetics, except at a very high stress level. Hydrogen content had a marked effect on time to fracture, also at the highest applied stress level. When hydrogen content was reduced by annealing, delayed cracking kinetics and susceptibility were suppressed, and cracking required a considerably higher stress level. The apparent critical hydrogen content, below which delayed cracking was not observed, was about 0.85 wppm. According to scanning electron microscope and electron backscattering diffraction examination, fracture mechanism in the constant load test specimens was mainly transgranular quasi‐cleavage, and cracking propagated along α′‐martensite.