新型节镍奥氏体不锈钢的成分设计及生产试制

 综合传统AISI300系奥氏体不锈钢和高氮奥氏体不锈钢的优缺点,开发一种新型节镍奥氏体不锈钢。为了研究其最优成分和工业试制工艺,通过Thermo-Calc热力学计算软件设计成分、热轧试生产试验钢和微观测试方法分析试验钢。结果表明,新型节镍奥氏体不锈钢最优的成分范围(质量分数)为N0.2%~0.3%、C小于0.1%、Cr 18%~20%、Mn 8%~10%、Ni 1%~2%,Fe余量。在1 240 ℃等温会析出大量的铁素体,以至于在1 150 ℃开轧时不能消除,出现裂纹,在改进热轧温度后,控制铁素体含量,得到无裂纹热轧卷。     

The Effect of Cold Rolling on Microstructure and Mechanical Properties of a New Cr–Mn Austenitic Stainless Steel in Comparison with AISI 316 Stainless Steel

In the present study the effect of room temperature rolling on microstructure and mechanical properties of a new Cr–Mn austenitic stainless steel (containing 12 %Cr, 23 %Mn and 0.13 %C) and AISI 316 steel was investigated. The specimens of these steels were cold rolled at various thickness reductions of 0, 12, 25, 37 and 50 %. Microstructural investigations were carried out using optical microscopy, magnetic field test and X-ray diffraction technique. Hardness and tensile test methods were also done to evaluate the mechanical properties. Results showed that some of austenite phase transformed to martensite during cold rolling in the 316 steel, while there was no strain induced transformation in the Cr–Mn steel. It was also found that the newly developed steel had higher strength and higher specific strength than those of the 316 steel, while its ductility was the same as that of the 316.     

Banded Microstructure Formation and Its Effect on the Performance of 06Ni9 Steel

Focusing on the banded microstructure formed during the production of 06Ni9 steels for cryo-LNG,this paper examines its formation,distribution of alloying elements,structure,hardness,and low-temperature property.The results show that the banded microstructure formed after hot-rolling and cooling of the steel binct in which the element segregation occurred during solidification.The phase change during heat treatment also can cause the formation of the banded microstructure of 06Ni9 steel.The white bands are mainly composed of ferrite and reversed austenite,and the black bands are mainly composed of reversed austenite and a certain amount of ferrite.Element segregation and formation of more carbide caused some black regions to appear.Grain refinement of 06Ni9steel is beneficial to the formation of reversed austenite,the redistribution of alloying elements,improving the stability of austenite and the low-temperature toughness of steel.This steel easily undergoes nickel segregation;thus,undergoing a secondary quenching and tempering process is recommended.The refinement of martensite quenching above A c3,the martensite that is rich in nickel and carbon,residual austenite and a few little of ferrite after secondary quenching lower than A c3 are beneficial to the formation of high stability austenite.Thus,this can meet the strength and toughness requirement of the low temperature 06Ni9 steel.     

冷作硬化非调质钢的连续冷却相变规律

钢的连续冷却相变曲线(CCT)是组织调控的基本依据,为了优化紧固件用冷作硬化非调钢热轧态的组织和力学性能,采用DIL805A相变仪测定了试验钢在0.1～50℃/s不同冷却速率下的热膨胀曲线,结合金相硬度法确定相变类型,并绘制了试验钢的CCT曲线.结果 表明,试验钢马氏体转变点(Ms)为280℃,在不同冷速范围内均有铁素...     

Study of strain induced martensite formation in a Ti modified 316 stainless steel bellow by transmission electron microscopy

This paper presents a study of strain induced martensite formed in a Ti modified SS 316 bellow by electron microscopy methods. This bellow is part of a bellows sealed valve used in a sodium testing facility. It is known that Ti is added to austenitic stainless steel to overcome the problem of intergranular corrosion due to sensitization. However, the addition of Ti, a strong ferrite stabilizer can influence the stability of γ phase especially under strained conditions. Detailed electron microscopy studies showed the presence of strain induced martensite in an austenite matrix. The reasons for its formation and stability at the operating temperature are discussed, since sensitization kinetics is reported to be accelerated by the formation of martensite.     

316H不锈钢铁素体的形成与控制

 为了提高316H不锈钢中厚板在钠冷快堆高温、强中子辐射环境下的质量稳定性,要求其残余铁素体面积分数不大于1%,而316H不锈钢中厚板的残余铁素体面积分数在常规工艺下为2%~8%。为了满足该要求,首先根据相图分析优化了316H不锈钢中C、Cr、Ni、Mo、N等元素含量,将其铬镍当量比控制到1.3以下,使其平衡态组织下铁素体面积分数小于7%。此外,根据相变过程,在凝固初期应快速冷却使钢水快速通过δ相区,尽可能少析出δ铁素体;在钢水快速通过δ相区后,应减缓冷却速度,使析出的铁素体尽可能多地通过包晶反应和高温扩散转变为奥氏体。因此,为了在316H连铸过程中实现这一目标,将钢水过热度从(45±5) ℃降低到(35±5)℃,铁素体面积分数最高由15%以上降低至10%附近;结晶器冷却水强度由2 700 L/min提高至3 000 L/min可以继续使铁素体面积分数从10%降至7%;最后再将二冷水比水量由0.75 L/kg降低至0.55 L/kg,整个连铸坯断面铁素体面积分数可全部降低至7%以下。通过连铸生产过热度、结晶器冷却强度、二冷水配水量3个工艺参数分步骤调整后得到的较低铁素体含量,连铸坯轧制成的钢板依然不能满足技术要求,需要将铸坯进行均质化处理,通过不同保温温度与保温时间的交叉试验,获得了最佳的均质化工艺,即1 250 ℃保温24 h,基本消除铸坯内残余铁素体组织,实现了残余铁素体面积分数不大于0.1%的316H中厚板的高效生产。     

Microstructural effects on fatigue crack growth in a low carbon steel

A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth. Formerly at the University of Connecticut     

The Effect of the Initial Microstructure on Recrystallization and Austenite Formation in a DP600 Steel

The effects of initial microstructure and thermal cycle on recrystallization, austenite formation, and their interaction were studied for intercritical annealing of a low-carbon steel that is suitable for industrial production of DP600 grade. The initial microstructures included 50 pct cold-rolled ferrite–pearlite, ferrite–bainite–pearlite and martensite. The latter two materials recrystallized at similar rates, while slower recrystallization was observed for ferrite–pearlite. If heating to an intercritical temperature was sufficiently slow, then recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The same trends as for recrystallization were found for the effect of initial microstructure on kinetics of austenite formation. The recrystallization–austenite formation interaction accelerated austenization in all the three starting microstructures by providing additional nucleation sites and enhancing growth rates, and drastically altered morphology and distribution of austenite. In particular, for ferrite–bainite–pearlite and martensite, the recrystallization–austenite formation interaction resulted in substantial microstructural refinement. Recrystallization and austenite formation from a fully recrystallized state were successfully modeled using the Johnson–Mehl–Avrami–Kolmogorov approach.     

Phase Evolution During the Liquid-Phase Bonding of Zirconium and Austenitic Stainless Steel with Zinc Insertion

Liquid-phase bonding experiments were performed at 1073?K (800?°C) between ZIRCALOY-2 and type 316 austenitic stainless steel by inserting zinc as an interlayer. The evolution of the microstructure at the interface was studied and the formation of various phases was detected. On the zirconium side, the very rapid formation of Zn₃Zr was detected, whereas on the steel side, an unexpectedly large amount of the base austenitic steel was observed to react with liquid Zn. The reacted iron solidified into a nickel-poor ferritic phase containing around 10?mol?pct?zinc, which grew into the austenite accompanied by a formation of a zinc-rich phase containing nickel. The reaction stopped when the zinc-rich phase reached saturation with a nickel content between 20 and 25?mol?pct. Thermodynamic calculations showed that the addition of nickel to liquid zinc greatly decreases the free energy of the liquid phase, thus enabling a large stability range for the ferrite?+?liquid zone and reducing the stability range of the austenite. The primary equilibrium between the austenite and the liquid phase is thus metastable, and thus, the austenite transforms into ferrite and a high-nickel-content liquid. The transformation front then progresses until ternary equilibrium is reached between austenite, ferrite, and the zinc-rich phase.     

Structure formation in grade 20 steel during equal-channel angular pressing and subsequent heating

The structure formation and the mechanical properties of quenched and tempered grade 20 steel after equal-channel angular pressing (ECAP) at various true strains and 400°C are studied. Electron microscopy analysis after ECAP shows a partially submicrocrystalline and partially subgrain structure with a structural element size of 340–375 nm. The structural element size depends on the region in which the elements are formed (polyhedral ferrite, needle-shaped ferrite, tempered martensite, and pearlite). Heating of the steel after ECAP at 400 and 450°C increases the fraction of high-angle boundaries and the structural ferrite element size to 360–450 nm. The fragmentation and spheroidization of cementite lamellae of pearlite and subgrain coalescence in the regions of needle-shaped ferrite and tempered martensite take place at a high ECAP true strain and heating temperature. Structural refinement ensures considerable strengthening, namely, UTS 742–871 MPa at EL 11–15.3%. The strength slightly increases, whereas the plasticity slightly decreases when the true strain increases during ECAP. After ECAP and heating, the strength and plastic properties of the grade 20 steel remain almost the same.     

Conversion Model for the Martensitic Transformation of Banded Austenite in a Ferrite Matrix

A model is proposed for the calculation of the volume fraction of martensite formed during the transformation of banded austenite in a hot-rolled AISI 430 stainless steel. The proposed model includes the strain resulting from the difference in coefficient of thermal expansion of austenite and ferrite and the effect of alloying elements on the lattice parameters. The model was verified by comparing the calculated values of the transformation strain of martensite with the experimental measurements.     

The effect of prior heat treatments on the structure and properties of warm-rolled AlSl 52100 steel

Isothermal rolling of AISI 52100 steel at 923 K (650 °C) has been used to produce very fine spheroidized carbide particles 0.1 to 0.2 μm in size in a submicron grain size ferrite matrix. Pearlitic starting structures are not completely spheroidized by such warm rolling and some isolated pearlite colonies remain after rolling. Tempered martensitic starting structures result in more uniform structures after rolling. The effect of varying the austenitizing time and temperature and of two cycle austenitizing treatments before rolling of tempered martensite were studied. After rolling, the room temperature yield strength was raised, a result of the finer dispersion of carbides and the associated reduction in ferrite grain size. The presence of isolated pearlite colonies in rolled material reduces the room temperature ductility. Formerly Graduate Student, Department of Mechanical Engineering, United States Naval Postgraduate School     

A layer growth mechanism proposed on studies on a gas nitrided AISI 316 L austenitic stainless steel

In this work an austenitic stainless steel AISI 316 L (1.4404) was gas nitrided at 540°C. The nitride layer was characterised by means of X-ray analysis, SEM, TEM and electron probe X-ray analysis. A fine lamellar structure comprising ? and γ' nitrides was formed. Nickel paritioned into γ'-Fe₄N type nitride while chromium partitioned into the ?-Fe_2–3N type nitride. A layer formation mechanism was proposed considering a local partition of the alloying elements Cr and Ni.     

Microstructure and tensile behavior of nitrogen-alloyed,dual-phase stainless steels

Two alloys of high-nitrogen stainless steel have been heat treated to produce dual-phase microstruc-tures. The first alloy, N10CrNiMol7 1, a Ni-containing stainless steel, was processed conventionally. The second alloy, N20CrMol7, a Ni-free stainless steel, was processed to obtain a higher nitrogen content by pressurized electroslag remelting. The martensite in N10CrNiMol7 1 was homogeneously distributed in the ferrite and obtained a near-constant volume fraction as a function of intercritical annealing temperature. Microprobe analysis and microhardness measurements of the martensite con-stituent suggested that up to 0.4 pct N was dissolved in the austenite before quenching. Austenite formation, martensite transformation, undissolved nitrides, and retained austenite were evaluated by transmission electron microscopy (TEM). The Ni-containing alloy exhibited classic dual-phase tensile behavior in that continuous yielding was observed together with good combinations of ultimate tensile strength and total elongation. The martensite constituent in alloy N20CrMol7 was concen-trated within bands. Comparison of tensile properties of the two alloys at similar volume fractions and hardness levels of martensite and ferrite showed that the microstructure containing banded mar-tensite had inferior combinations of strength and ductility. The degradation of tensile ductility was accompanied by a fracture mode transition from microvoid coalescence to transgranular cleavage. The deformation and fracture behavior of both alloys were related to the microstructure.     

Effect of heat input on microstructure and mechanical properties of dissimilar joints of AISI 316L steel and API X70 high-strength low-alloy steel

The microstructure and mechanical properties of dissimilar joints of AISI 316L austenitic stainless steel and API X70 high-strength low-alloy steel were investigated.For this purpose,gas tungsten arc welding(GTAW)was used in three different heat inputs,including 0.73,0.84,and 0.97 kJ/mm.The microstructural investigations of different zones including base metals,weld metal,heat-affected zones and interfaces were performed by optical microscopy and scanning electron microscopy.The mechanical properties were measured by microhardness,tensile and impact tests.It was found that with increasing heat input,the dendrite size and inter-dendritic spacing in the weld metal increased.Also,the amount of delta ferrite in the weld metal was reduced.Therefore,tensile strength and hardness were reduced and impact test energy was increased.The investigation of the interface between AISI 316L base metal and ER316L filler metal showed that increasing the heat input increases the size of austenite grains in the fusion boundary.A transition region was formed at the interface between API X70 steel and filler metals.     

Numerical Cooling Strategy Design for Hot Rolled Dual Phase Steel

In this article, a Mo‐Mn dual phase steel and its process parameters in hot rolling are discussed. The process window was derived by combining the experimental work in a hot deformation dilatometer and numerical calculation of the process parameters using rate law models for ferrite and martensite transformation. The ferrite formation model is based on the Leblond and Devaux approach while martensite formation is based on the Koistinen‐Marburger (K‐M) formula. The carbon enrichment during ferrite formation is taken into account for the following martensite formation. After completing the parameter identification for the rate law model, the evolution of phases in this steel can be addressed. Particularly, the simulations allow the prediction of preferable degree of retained strain and holding temperature on the run out table (ROT) for the required ferrite fraction.     

SEM Investigation of High‐Alloyed Austenitic Stainless Cast Steels With Varying Austenite Stability at Room Temperature and 100°C

The deformation mechanisms of high‐alloyed cast austenitic steels with 16% of chromium, 6% of manganese, and a nickel content of 3–9% were investigated by in situ and ex situ scanning electron microscopy. The austenite stability and the stacking fault energy were influenced by variation of the chemical composition as well as by changing deformation temperature (room temperature; RT and 100°C). The study shows that both an increase in austenite stability and stacking fault energy yield a significant change in the deformation mechanisms. Both increase of nickel content and increase in deformation temperature reduce the intensity of the martensitic phase transformation. Thus, the steel with low nickel content shows at RT pronounced formation of α′‐martensite. The steel with the highest nickel content, however, shows pronounced twinning.     

预处理组织对低碳钢IQ&P工艺下组织及性能影响

采用γ单相区和γ+α双相区轧制并淬火工艺以及双相区再加热-淬火-碳配分(IQ&P)工艺,研究预处理组织对低碳钢室温状态多相组织特征及力学性能的影响规律.实验用低碳钢经两种工艺轧制并淬火处理,获得马氏体和马氏体+铁素体的预处理组织,再经双相区IQ&P工艺处理后均获得多相组织.马氏体预处理钢的室温组织由板条状亚温铁素体、块状回火马氏体以及一定比例的针状未回火马氏体和8.2%的针状残余奥氏体组成;马氏体+铁素体预处理钢由板条状亚温铁素体、块状和针状未回火马氏体以及14.3%的短针状或块状残余奥氏体组成.在相同的双相区IQ&P工艺参数下,预处理组织为马氏体的钢抗拉强度为770 MPa,伸长率为28%,其强塑积为21560 MPa·%;而预处理组织为马氏体+铁素体的钢抗拉强度为834 MPa,伸长率增大到36.2%,强塑积达到30190 MPa·%,获得强度与塑性的优良结合.      

Dissimilar Spot Welds of AISI 304/AISI 1008: Metallurgical and Mechanical Characterization

This paper aims at investigating structure‐properties relationships in dissimilar resistance spot welding of AISI 304 austenitic stainless steel (SS) and AISI 1008 low carbon steel (CS). Microstructural characterization, microhardness test and the tensile‐ shear test were conducted. It was shown that the shape of the SS/CS fusion zone (FZ) is unsymmetrical and the final fusion line shifts from sheet/sheet interface into the higher resistivity side (i.e. AISI 304). FZ microstructure was ranged from ferrite‐austenite to full martensite depending on the dilution ratio of the base metals. The variation of SS/CS dissimilar welds failure mode was explained in terms of hardness/microstructure characteristics. It was concluded that to ensure pullout failure mode, welding parameters needed to adjust so that the FZ size is sufficiently large and dilution is sufficiently high to produce a martensite FZ. Fusion zone size at CS side proved to be the most important controlling factor of SS/CS peak load and energy absorption. Finally, the mechanical properties of SS/CS dissimilar welds were compared with SS/SS and CS/CS similar welds.     

Evaluation of Interface Boundaries in 9Cr-1Mo Steel After Thermal and Thermomechanical Treatments

The grain boundary character distribution (GBCD) and microstructure in 9Cr-1Mo ferritic/martensitic steel subjected to different heat treatments and thermomechanical treatments (TMTs) have been evaluated using electron backscatter diffraction (EBSD) technique. Microstructures obtained through displacive transformation of high-temperature austenite yielded higher amounts of Σ1-29 coincidence site lattice (CSL) boundaries (from 29 to 38 pct) compared with the ferrite grains obtained by diffusional transformation (~16 pct) or by recrystallization process (~14 pct). Specifically, the low-angle (Σ1), Σ3, Σ11, and Σ25b boundaries were enhanced in the tempered martensite substructure, whereas the prior austenite grain boundaries were largely of random type. Misorientation between the product ferrite variants for ideal orientation relationships during austenite transformation was calculated and compared with CSL misorientation to find its proximity based on Brandon’s criteria. The observed enhancements in Σ1, Σ3, and Σ11 could be interpreted based on Kurdjumov–Sachs (K–S) relation, but Nishiyama–Wassermann (N–W) relation was needed to understand Σ25b formation. The amounts of CSL boundaries in the tempered martensite structure were not significantly influenced by austenite grain size or the kinetics of martensitic transformation. In mixed microstructures of “polygonal ferrite + tempered martensite”, the frequencies of CSL boundaries were found to systematically decrease with increasing amounts of diffusional/recrystallized ferrite.