Weld decay-resistant austenitic stainless steel by grain boundary engineering

This paper presents an example of grain boundary engineering (GBE) for improving intergranular-corrosion and weld-decay resistance of austenitic stainless steel. Transmission and scanning electron microscope (TEM and SEM) observations demonstrated that coincidence site lattice (CSL) boundaries possess strong resistance to intergranular precipitation and corrosion in weld decay region of a type 304 austenitic stainless steel weldment. A thermomechanical treatment for GBE was tried for improvement of intergranular corrosion resistance of the 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small roll-reduction. The corrosion rate was much smaller in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface. The optimized 304 stainless steel showed an excellent resistance to weld decay during arc welding.     

Grain boundary engineering for improving stress corrosion cracking of 304 stainless steel

Grain boundary engineering (GBE) via low strain tension and annealing was used to enhance the resistance to stress corrosion cracking of a 304 stainless steel. Electron backscattered diffraction (EBSD) analysis exhibited that the GBE steel had a higher fraction of low-∑ coincidence site lattice (CSL) boundaries, larger grain-clusters, longer twin boundary chains, and fewer paths of connected non-twin boundaries with a more zigzag shape. Slow strain rate tests in high-temperature water showed that the GBE steel performed better plasticity, higher tensile strength, and similar yield strength compared to conventional steel. The low fraction of random boundaries in GBE steel resulted in a lower frequency of intergranular crack initiation, and the zigzag paths of non-twin boundaries made the intergranular crack propagation more difficult.     

Grain boundary engineering of 304 austenitic stainless steel by laser surface melting and annealing

In order to improve the intergranular corrosion resistance of 304 stainless steel, laser surface remelting experiments were conducted using a 2 kW continuous wave Nd: YAG laser. The grain boundary character distribution (GBCD) and microstructures of the materials were analyzed using EBSD, SEM and OM. The experimental results showed that combination of laser surface melting and annealing on 304 stainless steel resulted in a high frequency of twin boundaries and consequent discontinuity of random boundary network in the materials, which led to an improvement of resistance to intergranular corrosion. The maximum CSL density could reach 88.6% under optimal processing conditions: 1220 K and 28 h.     

16Cr奥氏体不锈钢晶间腐蚀的敏感性

为了研究1Cr17Mn6Ni5N奥氏体不锈钢(16Cr奥氏体不锈钢)的晶间腐蚀行为,通过光学显微镜(OM)、X射线衍射仪(XRD)和晶间腐蚀试验研究了其在不同敏化温度和冷却方式下,晶间碳化物的析出和耐晶间腐蚀性能的变化。结果表明:16Cr奥氏体不锈钢在敏化温度区间内加热时,晶界碳化物随加热温度的上升而增加,加热温度为850℃左右时晶界析出碳化物最多,主要为Cr_(23)C_6和Cr_7C_3;在敏化温度区间内相同加热温度时,水冷可显著减少其晶界碳化物的析出;16Cr奥氏体不锈钢对晶间腐蚀不敏感。     

S32750双相不锈钢相界与晶界特征对其力学性能和耐蚀性能的影响EI北大核心CSCD

通过固溶处理获得不同初始组织状态的S32750双相不锈钢样品,然后进行厚度压下量80%的冷轧变形和1050℃的退火处理,采用SEM-EBSD和XRD技术研究合金相界与晶界特征以及相组成分布情况,并利用拉伸实验、纳米压痕和双环电化学动电位再活化法(DL-EPR)分析不同初始状态样品的组织对力学性能与耐晶间腐蚀性能的影响规律。结果表明:高温固溶处理的合金样品经冷轧退火后晶粒细小均匀,两相分布接近1∶1,且相界占内界面(晶界+相界)比例较高,同相晶粒团簇程度最低,表现出优异的综合力学性能。合金样品经敏化处理后,σ相易沿α相晶界析出,高温固溶并经轧制退火后的样品中,由于α晶界比例较少且满足K-S取向关系的相界比例较高则又表现出良好的晶间腐蚀抗力。因此,通过适当的工艺来调控合金的相界与晶界分布可以实现材料强度和晶间腐蚀抗力的同步改善。     

Grain boundary character distribution and its effect on corrosion of Ni–23Cr–16Mo superalloy

Grain boundary character distribution (GBCD) of the Hastelloy C2000 alloy (Ni–23Cr–16Mo) and the effect of coincidence site lattice (CSL) grain boundaries on corrosion resistance were examined by electron backscattered diffraction and electrochemical experiments. Various deformation followed by annealing was applied to optimise the GBCD of the alloy. After grain boundary engineering (GBE) treatment, the proportion of CSL boundaries increased from 37.7% to 62.4% and the corrosion current density of the specimens decreased in NaCl solution. The results indicated that GBE treatment is responsible for preferable corrosion resistance due to the increase of the fraction of special low energy grain boundaries with perfect grain boundary atom arrangement after thermomechanical process.     

Study on microstructure of low carbon 12% chromium stainless steel in high temperature heat-affected zone

Coarsening, embrittlement and corrosion sensitization in high temperature heat-affected zone (HTHAZ) are the major problems when low carbon 12% chromium stainless steel is being welded, which induce deterioration of the impact toughness at low temperature and intergranular corrosion resistance. This study investigated the corresponding microstructures in HTHAZ with different chemical compositions and heat inputs through thermal simulation tests. There are several primary conclusions: (1) When ferrite factor (FF) is above 9.0, the microstructure in HTHAZ is fully ferrite or a small amount of martensite net likely distributing along delta ferrite grain boundaries. On the other hand, if FF is below 9.0, the martensite content increases with the decreasing of FF. (2) Heat input influences the microstructure of high FF steel in HTHAZ. The martensite content and its distribution of low FF steel are not sensitive to heat inputs, but the grain size grows up with the increase of heat inputs. (3) The coarse Ti-rich particles in low FF steels containing Ti can promote intragranular austenite formation inside delta ferrite resulting in packet morphology of martensite. On the other hand, martensite of low FF steels only stabilized with Nb is characterized by grain boundary allotriomorphs, Widmanstätten structures and secondary sawteeth. This martensite reticularly distributes along ferrite grain boundaries.     

316L-16MnR复合板不锈钢侧晶间腐蚀原因分析

采用GB／T4334．5—2000标准方法对不锈钢-低合金钢复合板进行晶间腐蚀试验,对不锈钢侧产生裂纹的原因进行了分析。结果表明,由于不锈钢板与低合金钢板的结合面上存在着不可抗拒的碳迁移现象,致使不锈钢板侧过渡区内的奥氏体晶界上聚集着大量的网状碳化物,从而造成晶界上的缺陷,使其在经过硫酸．硫酸铜溶液的腐蚀后,进行弯曲时产生了大量的晶间腐蚀裂纹。     

Improvement of Intergranular Stress Corrosion Crack Susceptibility of Austenite Stainless Steel through Grain Boundary Engineering

Intergranular stress corrosion crack susceptibility of austenite stainless steel was evaluated through threepoint bending test conducted in high temperature water. The experimental results showed that the frequent and efficient introduction of low energy coincidence site lattice boundaries through grain boundary engineering resulted in an apparent improvement of the intergranular stress corrosion crack resistance of austenite stainless steel.     

Grain size dependence of mechanical, corrosion and tribological properties of high nitrogen stainless steels

Austenitic stainless steels have been indispensable for the progress of technology during the last 80 years. Due to the cost of nickel and to the prospective of allergic reactions caused by this element, more and more laboratories and industries are trying to develop a new class of austenitic stainless steels with a low nickel content. In order to maintain the austenitic microstructure, nickel reduction is balanced with nitrogen addition. Nitrogen addition to austenitic stainless steels is also very effective for improving yield strength and corrosion resistance without reducing ductility and toughness. In order to further increase the strength, it is possible to combine the effect of nitrogen addition and grain refining. The purpose of this study is to examine the relationship between microstructures and mechanical, corrosion and tribological properties of a high nitrogen stainless steel with an ultrafine grained structure.     

Grain-size and heat-treatment effects in hydrogen-assisted cracking of austenitic stainless steels

Hydrogen embrittlement of 304L and 316L types austenitic stainless steels has been studied by charging thin tensile specimens with hydrogen through cathodic polarization. Throughout this study we have compared solution-annealed samples, having various prior austenite grain size, with samples given the additional sensitization treatment. The results of the tensile tests while undergoing cathodic charging show that the additional sensitization treatment and coarse-grained samples together, lower the mechanical properties in both 304L and 316L types, and the sensitized steel is more susceptible to hydrogen-assisted cracking. However, the room-temperature yield and ultimate strengths, and the elongation of type 316L, were much less affected depending on the heat treatment and prior austenitic grain size. The fracture surfaces of the specimens tested while cathodically charged show considerable differences between the annealed and the sensitized specimens. The sensitized coarse-grained specimens were predominantly intergranular in both 304L and 316L types, while the annealed 316L type specimens fracture shows massive regions of microvoid coalescence producing ductile rupture and the annealed 304L type specimens fracture were primarily transgranular and cleavage-like. Sensitization seems both to facilitate the penetration of hydrogen along the grain boundaries into the steel and to introduce susceptibility to fracture along grain boundaries while refined grain size improves resistance regardless of the failure mode.     

Effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel

The effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel was investigated in order to control fatigue crack propagation. The fraction of low-Σ coincidence boundaries in specimens was controlled by thermomechanical processing. The specimen with the higher fraction of low-Σ boundaries (73%) showed the lower propagation rate of fatigue crack than the specimen with the lower fraction of low-Σ boundaries (53%). The ratio of intergranular fracture segments to the total crack length was lower for the specimen with the higher fraction of low-Σ boundaries. Moreover, the roles of grain boundaries in the fatigue crack propagation were investigated in connection with grain boundary microstructure, i.e., the character distribution and geometrical configuration of grain boundaries. It is evidenced that the approach to grain boundary engineering is applicable to controlling fatigue crack propagation in austenitic stainless steel.     

Effect of stresses and strains on the intergranular corrosion of austenitic steels in nitric acid

We show that tensile stresses close to the yield limit and small strains (up to 5 %) facilitate an increase in the density of grain-boundary dislocations in sensitized austenitic steels 03Kh18N11, 08Kh18N10T, and 12Kh18N10T, create favorable conditions for repassivation of grain boundaries, and accelerate the intergranular corrosion (IC) of these steels in 50 % and 65 % solutions of nitric acid. The changes in the activation energy caused by stresses and strains indicate kinetic control of the dissolution of steel grain boundaries. This means that, in manufacturing and maintenance of stainless steel equipment used in nitric acid production, it is necessary to avoid stress concentration and reduce the IC susceptibility of the steels.Severodonetsk Scientific Research Institute of Chemical Mechanical Engineering, Severodonetsk. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 45–53, January–February, 1994.     

不锈钢晶间腐蚀弯曲评价方法的影响因素

用嵌含有GTN延性损伤模型的ABAQUS有限元法,模拟研究了不锈钢晶间腐蚀弯曲评价方法中材料力学性能、弯曲角度和压头直径对弯曲试样塑性应变分布、延性损伤和裂纹起裂的影响规律,分析了其对晶间腐蚀弯曲评价结果的影响。结果表明：随着试样弯曲角度的增大和弯曲压头直径的减小,试样拉伸面的塑性应变增加,试样越容易产生弯曲开裂;在晶间腐蚀弯曲评价标准中,当固定弯曲角度和压头直径时,对于塑性、韧性和抗断裂综合力学性能较低的不锈钢材料,在弯曲过程中材料本身会发生开裂;因此,需要考虑材料力学性能对晶间腐蚀弯曲评价结果的影响;对于该研究中的典型的奥氏体不锈钢材料,当其弯曲断裂应变低于0．51左右时,在弯曲过程中材料本身会发生开裂,不宜用弯曲方法来评价其晶间腐蚀敏感性。     

Suppression of chromium depletion and sensitization in austenitic stainless steel by surface mechanical attrition treatment

The effects of surface nanocrystallization via surface mechanical attrition treatment (SMAT) on degree of sensitization (DOS) of an austenitic stainless steel were investigated by means of double loop electrochemical potentiokinetic reactivation (DLEPR) test. The treated sample with grain size about 10 nm showed very low degree of sensitization value which can be considered as the non-sensitized material. This is mainly due to the formation of twin boundaries in the microstructure of the SMATed sample which weren't susceptible to carbide precipitation because of their regular and coherent atomic structure and extreme low grain boundary energy as compared with those of other grain boundaries.     

A review on nickel-free nitrogen containing austenitic stainless steels for biomedical applications

The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life. Metallic materials are often used as biomaterials to replace structural components of the human body. Stainless steels, cobalt–chromium alloys, commercially pure titanium and its alloys are typical metallic biomaterials that are being used for implant devices. Stainless steels have been widely used as biomaterials because of their very low cost as compared to other metallic materials, good mechanical and corrosion resistant properties and adequate biocompatibility. However, the adverse effects of nickel ions being released into the human body have promoted the development of “nickel-free nitrogen containing austenitic stainless steels” for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel and emphatically the advantages of nitrogen in stainless steel, as well as the development of nickel-free nitrogen containing stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength, better corrosion and wear resistance and superior biocompatibility in comparison to the currently used austenitic stainless steel (e.g. 316L), the newly developed nickel-free high nitrogen austenitic stainless steel is a reliable substitute for the conventionally used medical stainless steels.     

Influence of heat treatment on creep of a Mn–N stabilised austenitic stainless steel

Creep at 700 °C/196 MPa and 900 or 925 °C/27.4 MPa of 21Cr–4Ni–9Mn austenitic stainless steel is determined as a function of the heat treatment. The heat treatment variation involves altering the solution heat treatment cooling rate from water quenching to cooling at 6 or 4 °C/min causing: serrated grain boundaries versus planar grain boundaries, coarser intergranular carbides, and discontinuous precipitation of grain boundary reaction zones. Water quenching causes improved creep resistance. Creep fracture and cracking is intergranular. Coarse intergranular carbides and grain boundary reaction zones cause premature void formation and cracking, this damage leading to an accelerating creep rate and lowering creep resistance of the more slowly cooled conditions. During creep, grain boundary serrations, which may otherwise contribute to improved creep, are eliminated. Determining the individual influence of grain boundary serrations on creep requires a detailed investigation of various heat treatment parameters to prevent concurrent formation of grain boundary reaction zones and serrations.     

Intergranular corrosion of a stud used in safety relief valve

In the present paper, a failure case study on intergranular corrosion of stainless steel stud is discussed. The stud was in the assembly under marine environment, for a period of 11 years. Detailed investigation revealed that the crack had propagated in an intergranular manner, after initiation at threaded region of the stud, leaving behind widespread grain dissolution. The presence of chloride ions in deposits on central region of stud, exposed to marine environment was also established. The high carbon content, coupled with exposure to sensitization range of stainless steel during processing caused corrosion along intergranular corrosion.This paper brings out the details of investigation carried out on the failed stud.     

奥氏体不锈钢在石油精炼环境中应力腐蚀破裂

通过恒变形和恒载荷试验研究了敏化18—8型奥氏体不锈钢在连多硫酸介质中的应力腐蚀行为。研究表明,介质浓度在2.0～6.0％,PH 值在0.8～1.8范围是材料出现应力腐蚀破裂的敏感介质。断口分析表明,在该体系中破裂属晶间型。钢中晶界贫铬区的存在是产生晶间型应力腐蚀破裂的主要原因。     

Effect of single-step strain and annealing on grain boundary character distribution and intergranular corrosion in Alloy 690

The effects of single-step thermomechanical treatments on the grain boundary character distribution (GBCD) and intergranular corrosion of Alloy 690 (Ni–30Cr–10Fe, wt.%) are investigated. High proportion of low ΣCSL grain boundaries (more than 70% according to Palumbo–Aust criterion) associating with large size grains-cluster microstructure is obtained through one-step thermomechanical treatment of 5% cold rolling followed by annealing at 1,100 °C for 5 min. Nucleation density of recrystallization and multiple twinning are the key factors affecting the GBCD. The grains-cluster is produced by multiple twinning starting from a single recrystallization nucleus. That the mean size of the grains-clusters and proportion of low ΣCSL boundaries decrease with the increasing strain, is caused by the increasing nucleation density of recrystallization with the increase of strain. The specimen with large size grains-cluster microstructure and high proportion of low ΣCSL boundaries exhibits much better resistance to mass loss during intergranular corrosion testing than that with small size grains-cluster microstructure and relatively low proportion of low ΣCSL boundaries.