Hydrogen-related phase transformations in austenitic stainless steels

The effect of hydrogen and stress (strain) on the stability of the austenite phase in stainless steels was investigated. Hydrogen was introduced by severe cathodic charging and by elevated temperature equilibration with high pressure H₂ gas. Using X-ray diffraction and magnetic techniques, the behavior of two “stable” type AISI310 steels and an “unstable” type AISI304 steel was studied during charging and during the outgassing period following charging. Transformation from the fcc γ phase to an expanded fcc phase, γ*, and to the hcp ε phase occurred during cathodic charging. Reversion of the γ* and e phases to the original γ structure and formation of the bcc α structure were examined, and the kinetics of these processes was studied. The γ* phase was shown to be ferromagnetic with a subambient Curie temperature. The γ⇆ε phase transition was studied after hydrogen charging in high pressure gas, as was the formation of a during outgassing. These results are interpreted as effects of hydrogen and stress (strain) on the stability of the various phases. A proposed psuedo-binary phase diagram for the metal-hydrogen system was proposed to account for the formation of the γ* phase. The relation of these phase changes to hydrogen embrittlement and stress corrosion cracking of stainless steel is discussed.     

Microstructural origin of the skeletal ferrite morphology of austenitic stainless steel welds

Scanning transmission electron microscopy (STEM) was conducted on welds exhibiting a variety of skeletal, or vermicular ferrite morphologies in addition to one lathy ferrite morphology. These ferrite morphologies result from primary ferrite solidification followed by a solid state transformation upon cooling. During cooling, a large fraction of the ferrite transforms to austenite leaving a variety of ferrite morphologies. Comparison of composition profiles and alloy partitioning showed both the skeletal and lathy ferrite structures result from a diffusion controlled solid state transformation. However, the overall measured composition profiles of the weld structure are a result of partitioning during both solidification and the subsequent solid state transformation.     

Grain refinement of wrought austenitic stainless steels by rapid heating

Rapid heating after cold deformation has been investigated as a grain refinement technique for austenitic stainless steels. The method shows considerable promise for obtaining average grain diameters in the 3 to 10 μm range without causing carbide precipitation in the alloy. This technique has been used here to obtain a wide range of grain sizes in type 316 steel for property measurements. The hardness, tensile strength and 0.2 pct offset yield strength were found to vary linearly with the reciprocal of the square root of the average grain diameter.     

Creep-fatigue interaction in austenitic stainless steels

Low cycle fatigue failures occur by the initiation and controlled growth of a surface crack. The development of crack propagation models, based on continuum mechanics, have enabled successful predictions of fatigue life at both room and elevated temperatures. This paper attempts to extend such models to cover the situations in which creep damage, introduced during periods of stress relaxation, influences the rate of growth of the surface fatigue crack. Equations predicting fatigue life as a function of hold period are in good agreement with experimental data, for Type 304 stainless steel, Type 316 stainless steel and Incoloy-800.     

Instabilities in stabilized austenitic stainless steels

The effect of aging on the precipitation of grain boundary phases in three austenitic stainless steels (AISI 347, 347AP, and an experimental steel stabilized with hafnium) was investigated. Aging was performed both on bulk steels as well as on samples which were subjected to a thermal treatment to simulate the coarse grain region of the heat affected zone (HAZ) during welding. Aging of the bulk steels at 866 K for 8000 hours resulted in the precipitation of Cr₂₃C₆ carbides, σ, and Fe₂Nb phases; the propensity for precipitation was least for the hafnium-stabilized steel. Weld simulation of the HAZ resulted in dissolution of the phases present in the as-received 347 and 347AP steels, leading to grain coarsening. Subsequent aging caused extensive grain boundary Cr₂₃C₆ carbides and inhomogeneous matrix precipitation. In addition, steel 347AP formed a precipitate free zone (PFZ) along the grain boundaries. The steel containing hafnium showed the best microstructural stability to aging and welding. Formerly with Exxon Research and Engineering Company.     

Comparison of hydrogen gas embrittlement of austenitic and ferritic stainless steels

Hydrogen-induced slow crack growth (SCG) was compared in austenitic and ferritic stainless steels at 0 to 125 °Cand 11 to 216 kPa of hydrogen gas. No SCG was observed for AISI 310, while AISI 301 was more susceptible to hydrogen embrittlement and had higher cracking velocity than AL 29-4-2 under the same test conditions. The kinetics of crack propagation was modeled in terms of the hydrogen transport in these alloys. This is a function of temperature, microstructure, and stress state in the embrittlement region. The relatively high cracking velocity of AISI 301 was shown to be controlled by the fast transport of hydrogen through the stress-induced α′ martensite at the crack tip and low escape rate of hydrogen through the γ phase in the surrounding region. Faster accumulation rates of hydrogen in the embrittlement region were expected for AISI 301, which led to higher cracking velocities. The mechanism of hydrogen-induced SCG was discussed based upon the concept of hydrogen-enhanced plasticity. Formerly Research Associate of the University of Illinois     

Ferritic-austenitic solidification mode in austenitic stainless steel welds

The macro-and microstructures of about fifty different stainless welds of the AISI/ AWS 300 series are analyzed. The results indicate that under conditions corresponding to a typical shielded metal arc (SMA) welding the welds with a ratio in the range 1.48≾Cr _eq /Ni _eq ≾1.95, where Ni _eq and Cr _eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify in accordance with a duplex mode with the delta ferrite as the primary (leading) phase. The austenite forms between ferrite dendrites through a three-phase reaction between liquid, ferrite and austenite, and subsequently grows into the ferrite by either an equiaxial or an acicular mechanism, resulting in a drastic decrease in the volume fraction of the delta ferrite. The micro-structure at room temperature is characterized by a general irregularity and the varied morphology of the ferrite. The compositional differences observed at room temperature are a consequence both of the solidification and the solid state transformation. Formerly Research Staff Member, Laboratory of Physical Metallurgy, University of Oulu.     

Austenitic solidification mode in austenitic stainless steel welds

The micro- and macrostructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that in welding condition corresponding to a typical SMA welding those and only those welds in which the ratio Cr_eq/Ni_eq≲1.48, where Ni_eq and Cr_eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify with the austenite as the primary or leading phase and the delta ferrite, if any, formed from the rest melt between growing cells or cellular dendrites of the austenite. At room temperature these welds are characterized by a regular general microstructure, soft forms of the ferrite and relatively large compositional differences mainly caused by solidification. T. TAKALO, formerly Research Staff Member, University of Oulu     

Microstructural and microchemical aspects of the solid-state decomposition of delta ferrite in austenitic stainless steels

The solid-state decomposition of delta-ferrite in a 20-10 stainless steel has been followed using optical microscopy, transmission electron microscopy, and high-resolution STEM microanalysis. As the quenching rate was increased, the formation of Widmanstätten austenite was replaced by a plane-front diffusional transformation, with near local equilibrium at the austenite-ferrite interface. Retained ferrite rims, observed at the impingement points of austenite grains, were used to characterize the nature of this transformation. Recent analyses of the thermodynamics of the massive transformation reaction are used to discuss the results.     

Hydrogen induced ductility losses in austenitic stainless steel welds

The effect of hydrogen on the tensile behavior of austenitic stainless steel welds was studied in two AISI 300 series alloys and two nitrogen strengthened alloys. The microstructure of these welds typically contained several percent ferrite in an austenite matrix. Hydrogen was found to reduce the ductility of all welds; however, the severity of ductility loss increased with increasing tendency to deform via a planar slip mode. In materials exhibiting large degrees of slip planarity, 304L and 308L, hydrogen changed the fracture mode from dimple rupture to a mixed mode of ductile and brittle fracture associated with the austenite-ferrite interface. The two alloys, 22-13-5 and 309S, which tend to deform by cross slip mechanisms, showed smaller losses in ductility even though hydrogen assisted the ductile rupture process by aiding void growth and coalescence, without changing the fracture mode. Varying the amount of ferrite from approximately one to 10 pct had no significant effect on performance in hydrogen.     

High temperature phase chemistries and solidification mode prediction in nitrogen-strengthened austenitic stainless steels

Nitronic 50 and Nitronic 50W, two nitrogen-strengthened stainless steels, were heat treated over a wide range of temperatures, and the compositions of the ferrite and austenite at each temperature were measured with analytical electron microscopy techniques. The compositional data were used to generate the (γ + δ phase field on a 58 pct Fe vertical section. Volume fractions of ferrite and austenite were calculated from phase chemistries and compared with volume fractions determined from optical micrographs. Weld solidification modes were predicted by reference to the Cr and Ni contents of each alloy, and the results were compared with predictions based on the ratios of calculated Cr and Ni equivalents for the alloys. Nitronic 50, which contained ferrite and austenite at the solidus temperature of 1370 °C, solidified through the eutectic triangle, and the weld microstructure was similar to that of austenitic-ferritic solidification. Nitronic 50W was totally ferritic at 1340 °C and solidified as primary delta ferrite. During heat treatments, Nitronic 50 and Nitronic 50W precipitated secondary phases, notably Z-phase (NbCrN), sigma phase, and stringered phases rich in Mn and Cr.     

氮含量对无镍奥氏体不锈钢平衡相转变的影响

高氮铬锰奥氏体不锈钢有着极为广泛的应用前景,然而氮含量对其相转变的影响尚不十分清晰。设计并冶炼了氮质量分数为0.02%～1.20%的试验钢,对各钢的平衡相转变进行了热力学计算,对δ铁素体和Cr₂N的形貌进行了观察。结果表明,钢中δ铁素体的最大析出量随着氮含量的升高而降低,当氮质量分数超过1.05%后,无δ铁素体析出。获得了试验钢加热时δ铁素体的析出温度与氮含量的关系式。随着氮含量的升高,试验钢在冷却时Cr₂N的析出温度逐渐升高,并获得了其定量关系式。在GN04钢中,1 200 ℃等温2 h后的δ铁素体主要沿三叉晶界分布。Cr₂N析出优先在晶界形成,然后朝着晶内发展。在相同等温条件下,试验钢中Cr₂N的析出量随着氮含量的升高而增大,且层片间距随之减小。     

Mn对2205双相不锈钢耐点蚀性能的影响

双相不锈钢2101生产成本低,性能优异,近年来被逐渐重视。采用“电炉+AOD+模铸”的工艺生产2101双相不锈钢,在AOD精炼过程中,研究了温度和主要成分Cr、C、Si的变化情况,结果显示,AOD炉有很好的脱碳效果,能将C质量分数（w_[C]）由2.5%脱至0.03%以下,在还原期,Si对Cr有很好的还原效果。精炼过程中,最重要的是脱碳,但将碳脱至0.1%以后,所需要的条件变得苛刻。通过热力学计算公式,研究了双相不锈钢2101去碳保铬的影响因素,结果表明,碳铬平衡主要受CO分压和温度的影响,CO分压越低、温度越高越有利于脱碳。在CO分压一定,w_[C]<0.1%时,w_[C]越低,碳铬平衡曲线的斜率越大,脱碳需要的温度越高,脱碳越困难,降低CO分压可进一步脱碳。在P_CO/P₀= 0.4,w_[Cr]=21.5%条件下,为将w_[C]脱至0.03%以下,需要将炉内温度升到1746.1 ℃以上。     

铸造奥氏体不锈钢的疲劳裂纹扩展

用紧凑拉伸试样研究了载荷比、单峰过载和两步高-低幅加载对Z3CN20-09M铸造奥氏体不锈钢疲劳裂纹扩展速率的影响.当应力强度因子范围相同时,疲劳裂纹扩展速率随载荷比的增大而增大.单峰过载使裂纹扩展速率先有短暂的增加后长距离的减速扩展,出现裂纹扩展迟滞现象.两步高-低幅加载时,若两步的最大载荷不同,第二步裂纹扩展也会出现迟滞现象.用两参数模型和Wheeler模型能够预测恒幅载荷和变幅载荷下的疲劳裂纹扩展行为.     

Sliding wear of austenitic and austenitic-ferritic stainless steels

The dry sliding behavior of a 304L austenitic stainless steel and a duplex 2205 austenitic-ferritic stainless was investigated. The evolution of wear was characterized by the existence of a sliding-distance transition. In particular, wear passed from delamination to tribo-oxidation, with a reduction in wear rate. The occurrence of such a transition was interpreted with reference to a theory of sliding wear based on the formation, by subsurface plastic deformation, of a tribological layer and its detachment during sliding. It has been found that the transition is controlled by the ability of the tribological system to form, on its outer part, a protective oxide-rich scale. This introduces a kinetic limitation, which is particularly important in the case of the duplex 2205, because of its lower ductility in comparison to the 304L steel. In this frame, the influence of sliding velocity, the particular frictional behavior, the role of chromium in the oxidative wear, and the surface temperature evolution during sliding could be explained.     

固溶时效后高氮奥氏体不锈钢的物理化学相分析

采用物理化学相分析技术研究了高氮奥氏体不锈钢固溶时效后的碳、氮化物析出行为,确定了析出相的类型、粒度分布、含量及组成结构式.结果表明:氮含量低的1Cr22Mn15N0.6以M₂₃C₆型碳化物析出为主,氮含量高的1Cr22Mn15N0.9以(CrFe)₂N_1-x型氮化物析出为主,高氮奥氏体不锈钢中析出物的总量随时效时间的延长而增加.