Microstructure and mechanical properties of austenitic stainless steels after cold rolling

The effect of austenite stability on the evolution of microstructure and mechanical properties of three austenitic stainless steels during cold rolling has been studied. Samples of different grain sizes have been used to characterize the microstructures during deformation. In the case of 304/8% Ni and 304/10% Ni stainless steels, the transformation microstructures consist of mechanical twins: ε-martensite and α′-martensite. No hexagonal close-packed (hcp) ε-martensite was detected in 316 stainless steel. The volume fraction of α′-martensite formed increases with increasing strain in 304 and 316 stainless steels for a given grain size. The amount of α′ phase increases with a decrease in grain size in 304 stainless steel, while the formation of this phase has been found to be grain size insensitive in 316 stainless steel. The strain-hardening behavior exhibited by the three stainless steels used in this study indicates the contribution of both α′-martensite and grain size strengthening in the case of both 304 stainless steels, while only grain size contribution was found in the case of 316 stainless steel.     

Pitting and Intergranular Corrosion Resistance of AISI Type 301LN Stainless Steels

The pitting and intergranular corrosion (IGC) resistance of AISI type 301LN stainless steels were evaluated using ASTM methods, anodic polarization, and electrochemical impedance techniques. The IGC results indicated that the microstructure of the samples after sensitization heat treatment at 675 °C for 1 h shows step or dual structure for both imported and indigenous materials indicating insignificant Cr₂₃C₆ precipitation. The results of immersion tests in boiling 6% copper sulfate + 16% sulfuric acid + copper solution for 24 h followed by the bend test (ASTM A262 Practice-E method) indicated no crack formation in any of the tested specimens. Pitting corrosion resistance carried out in 6% FeCl₃ solution at different temperatures of 22 ± 2 and 50 ± 2 °C (ASTM G 48) up to the period of 72 h revealed pitting corrosion attack in all the investigated alloys. The potentiodynamic anodic polarization results in 0.5 M NaCl revealed variation in passive current density and pitting potential depending on the alloy chemistry and metallurgical condition. The passive film properties studied by electrochemical impedance spectroscopy (EIS) correlated well with the polarization results. The x-ray diffraction (XRD) results revealed the presence of austenite (γ) and martensite (α′) phases depending on the material condition. The suitability of three indigenously developed AISI type 301LN stainless steels were compared with imported type 301LN stainless steel and the results are highlighted in this article.     

Sintered stainless steels: Fatigue crack propagation resistance under hydrogen charging conditions

Monophasic and multiphasic (two and three phases) sintered stainless steels were prepared both considering premixes of AISI 316LHC and AISI 434LHC stainless steels powders and using a prealloyed duplex stainless steel 25% Cr, 5% Ni, 2% Mo powder. Their fatigue crack propagation resistance was investigated both in air and under hydrogen charging conditions (0.5 M H₂SO₄ + 0.01 M KSCN aqueous solution; applied potential = −700 mV/SCE), considering three different stress ratios (R = 0.1; 0.5; 0.75). Fatigue crack propagation micromechanisms were investigated by means of fracture surface scanning electron microscope (SEM) analysis.For all the investigated sintered stainless, fatigue crack propagation resistance is influenced by hydrogen charging and an increase of crack growth rates dependent on the steel microstructure is obtained. Experimental results also allow to identify the sintered stainless steel obtained from the prealloyed 25% Cr, 5% Ni, 2% Mo powder as the most resistant to fatigue crack propagation in air and under hydrogen charging conditions.     

Effect of Residual Stress on Fatigue Failure of Carbonitrided Low-Carbon Steel

The effect of residual stress on fatigue behavior and mechanisms of carbonitrided AISI 1015 steel under uniaxial cyclic loading has been experimentally studied. By progressive removal of thin surface layers using an electropolishing technique and subsequent residual stress measurements using an x-ray diffraction technique, the compressive residual stress at the surface was approximately 900 MPa. The stress decreased toward the center, and became stable tensile residual stress of approximately 20 MPa. The fatigue resistance of carbonitrided AISI 1015 steel was higher than that of AISI 1015 steel due to the presence of compressive residual stress in case layer. The fatigue limit of AISI 1015 steels with and without carbonitriding was 340 and 300 MPa, respectively. Subsurface cracks initiated at the case-core interface, i.e. approximately 400 μm from the surface. With increasing number of stress cycles, the subsurface cracks coalesced and propagated intergranularly through the case layer. After some incubation cycles, the subsurface cracks reached the surface of specimen, and became a main crack. During this stage, the stress increased, and caused the formation of voids in core material. Consequently, the crack propagated through the core material, interacted with voids, and caused complete fracture.     

Deformation-plastic behavior of highly irradiated stainless steels 12Kh18N10T and 08Kh16N11M3 at cryogenic and elevated temperatures

In the temperature range from −115 to +120°C, mechanical tests of samples of austenitic stainless steels 12Kh18N10T and 08Kh16N11M3—materials for the casings of spent fuel assemblies of a reactor BN-350 irradiated by neutrons to damaging doses of 11–55 dpa—have been carried out. In the range of cryogenic temperatures, in these highly irradiated steels there was discovered and investigated a new phenomenon—a “wave of plastic deformation,” which consists in the initiation, development, and propagation of a deformation band over the length of the sample, which leads to a possibility of reaching total relative deformation of 20% and greater, instead of 3–5% usually observed at given damaging doses. The role of a martensitic γ → α′ transformation in the formation of the “wave” and in the improvement in the mechanical properties of the metastable steel irradiated by neutrons is discussed.     

Corrosion reactivity of laser-peened steel surfaces

Laser-shock processing or laser peening (LP) is a novel process used to reinforce surfaces by generating compressive residual stresses that has been investigated to change the surface mechanical state and modify the electrochemical properties of three commercial steels. The first part of this paper relates to experiments where LP has been applied to G10380 and G41400 steels for corrosion testing in an acid HKSO₄-0.3 M solution. Only in the case of G41400 martensitic steel is a reduction of the corrosion current observed, depending on the degree of work hardening and the amplitude of compressive stresses. This indicates a small mechanochemical effect of LP, which seems to be restricted to martensitic structure. Second, the effect of LP on stress corrosion cracking (SCC) of AISI 316L stainless steel is demonstrated by static tests in MgCl₂ 44% – 153 °C solution. The results confirm the applicability of LP to suppress cracks on all the areas processed without occurrence of any problems in the treated-nontreated transitions zones.     

Effect of austenite instability on the hydrogen-enhanced crack growth of austenitic stainless steels

Fatigue crack growth tests were performed to assess the fatigue behavior of AISI 316L and 254 SMO stainless steels (SSs) in air and gaseous hydrogen. 254 SMO SS generally exhibited a greater resistance to fatigue crack growth than 316L. Sensitization treatment had only a marginal effect on the fatigue crack growth behavior of both alloys in air. Moreover, 316L SS exhibited significant hydrogen-enhanced crack growth but 254 SMO, even sensitized 254 SMO specimens, did not. A thin layer of strain-induced martensite was formed on the fatigue-fractured surface of the 316L SS, and its content increased when raising the stress ratio. The thin martensite layer was responsible for the hydrogen-enhanced fatigue crack growth of the 316L SS. By contrast, the extremely stable austenite was responsible for the low susceptibility of 254 SMO SS to hydrogen-accelerated crack growth. The trapping of hydrogen at the grain boundaries and the transformed martensite in the sensitized 316L specimens led to increased fatigue crack growth rates and intergranular fracture of the material.     

Mössbauer Effect Study of Room Temperature Cathodic Polarization of AISI321SS Austenitic Stainless Steel

Room temperature hydrogen charging by cathodic polarization of cold rolled AISI 321SS austenitic stainless steel in appropriate electrolytic medium leads to its decomposition to structural defects and a ferromagnetic α′-martensitic phase. The degree of decomposition, and hence the resulting products depends on hydrogen charging time with martensitic transformation yielding up to 14-22% martensite for charging periods of 30 and 96 h, respectively. Based on M?ssbauer spectroscopy measurements, the magnetically split portion of the spectra corresponding to the α′-martensite phase was resolved in terms of one Fe-site with internal magnetic field in the range of 260-265 ± 10 kOe. Both the uncharged and retained (after hydrogen charging) austenitic phases were resolved similarly at ambient and sub-ambient cryogenic temperatures. The austenitic phase in both the uncharged and charged states remained stable from ambient down to 4.2 K, where they exhibited singlet broadening suggesting weakly ferro/antifero-magnetic ordering.     

Fatigue life prediction using two stage model for AISI 304L cruciform joints,with different fillet geometry,failing at toe

Abstract

Reports in the literature indicate that the fillet geometry affects the fatigue properties of cruciform welded joints in structural steels. In an attempt to study the above effect with respect to stainless steel sheet metal welded joints, load carrying transverse fillet welded cruciform joints having a two fillet geometry were fabricated from AISI 304L stainless steel using gas tungsten arc welding and gas metal arc welding with 308L electrodes. The objective of the present work is to predict the fatigue life of AISI 304L cruciform joints failing at the weld toe using a two stage model. The local stress life method was applied to calculate the fatigue crack initiation life, whereas the fatigue crack propagation life was estimated using fracture mechanics concepts. Constant amplitude fatigue tests with stress ratio R ~ 0 were carried out using a 100 kN servohydraulic Dartec universal testing machine at a frequency of 30 Hz. An automatic crack monitoring system based on crack propagation gauges was used to obtain the propagation data during the fatigue process. The predicted lives were compared with the experimental values.     

喷丸强化残余应力对AISI304不锈钢疲劳裂纹扩展行为的影响

目的 探究喷丸强化残余压应力对AISI 304不锈钢疲劳裂纹扩展行为的影响规律.方法 建立并联合紧凑拉伸(CT)试样三维有限元模型和对称胞元喷丸有限元模型,发展一套多步骤数值模拟方法.首先,建立AISI 304不锈钢CT试样的三维有限元模型,模拟不同外加交变载荷工况下的疲劳裂纹扩展过程.基于线弹性断裂力学理论,利用裂纹闭合技术,计算不同裂纹长度对应的应力强度因子范围,采用修正的Paris公式计算疲劳裂纹扩展速率,并通过试验数据对计算结果进行考核.其次,建立多弹丸分层逐次冲击靶面的对称胞元喷丸有限元模型,模拟100％和200％喷丸覆盖率下的残余应力场,并通过试验数据对该对称胞元喷丸有限元模型的有效性进行验证.最后,将喷丸强化诱导的残余应力场以读写外部文件的方式导入CT试样三维有限元模型,模拟在内部残余应力场和外部交变载荷共同作用下的疲劳裂纹扩展行为.结果 对于相同的喷丸工况,保持外加载荷比不变而减小最大外加载荷,或者保持最大外加载荷不变而减小外加载荷比,喷丸强化诱导的残余压应力对疲劳裂纹扩展的抑制作用愈加显著.对于相同的外加载荷工况,200％喷丸覆盖率工况比100％喷丸覆盖率工况更能有效降低AISI 304不锈钢的疲劳裂纹扩展速率.结论 喷丸强化诱导的残余压应力场能够有效抑制AISI 304不锈钢的疲劳裂纹扩展.     

Effect of cooling rate on plasma nitrocarburised compound layers for pure iron

Plasma nitrocarburising was carried out with various CO gas compositions with 3 mbar at 570°C for 3 hours. After treatment, different cooling rates (fast cooling, slow cooling and gas cooling) were used to investigate their effect on the structure of the compound layer, the ε phase being the outer part of the compound layer and the γ′ phase being the inner part The CO gas composition influenced the constitution of the compound layer produced, i.e. high CO contents were essential for the production of an ε phased compound layer, as did a variation of the cooling rate, with a large amount of γ′ phase transformed from the ε phase during slow cooling. The transformation was reduced with an increasing cooling rate and prevented by a fast cooling rate of approximately 18.5°C/sec at 500°C, at which temperature this transformation began.     

The effect of sensitization on the hydrogen-enhanced fatigue crack growth of two austenitic stainless steels

Fatigue crack growth tests were performed to evaluate the susceptibility to hydrogen-enhanced crack growth of AISI 304 and 316 stainless steels (SSs). Sensitization treatment at 650 °C 100 h played little effect on the fatigue crack growth behavior in air, regardless of testing specimens. However, hydrogen accelerated the fatigue crack growth of various specimens to different degrees; sensitized specimens were more susceptible as compared with the un-sensitized ones.

Fatigue fracture appearance of various specimens tested in air exhibited mainly transgranular fatigue fracture together with rarely intergranular fracture and twin boundary separation. Meanwhile, intergranular fracture was found for sensitized specimens tested in hydrogen. Extensive quasi-cleavage fracture related to the strain-induced martensite accounted for the hydrogen-accelerated fatigue crack growth of unstable austenitic SSs. On the other hand, the lower susceptibility of 316H specimens could be attributed to the partial austenite transformation, as evidenced by a mixture of transgranular fracture feature and quasi-cleavage.     

Untersuchung des SwRK-Verhaltens des hochreinen martensitaushärtenden Stahles X5CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X4CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 2: SwRK-Untersuchungen und Rißeinleitungsmechanismen

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel X5CrNiCuNb 17 4 PH in comparison to the soft martensitic stainless steel X4CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 2: Corrosion fatigue tests and crack initiation mechanisms The following report concerns the study of the corrosion fatigue behaviour of the soft martensitic steel X4CrNiMo 16 5 1 ESR and the precipitation hardened X5CrNiCuNb 17 4 PH in sodium solution in the temperature range between 20° and 150 °C and the determination of their general corrosion properties and the mechanism of crack propagation. Their corrosion fatigue limits were compared with each other. A comparision was also made between an electro-slag-remelted soft martensitic steel and a charge without an ESR aftertreatment. Microfractographical fracture and crack path investigation were carried out for interpretation of the experimental results. It was observed that in both super pure steels (soft martensitic and precipitation hardened) the oxidic inclusions are not responsible for the crack intiation, as it was found in the non ESR treated steels. In the 17-4 PH steel copper containing inclusions in the crack initiation areas were observed. In concentrated sodium solution pitting corrosion was found at both steels.     

Phenomena and mechanism on superplasticity of duplex stainless steels

The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after being solution treated at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10^-3-10^-1s^-1. The microstructure of duplex stainless steel consists of a matrix γ phase having low angle grain boundaries and a σ phase as second phase particles before the deformation at 850°C. It is well known that the constituent phases in duplex stainless steel is changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during deformation at 850°C. The final microstructure of duplex stainless steel consisted of 70 vol.% of γ and 30 vol.% of the σ phase. A maximum elongation of 750% was obtained at 850°C with a strain rate of 3.16xl0^-3s^-1. The dislocation density within matrix γ grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring γ grains increased as the strain increased, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by dynamic recrystallization during the deformation at 850°C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850°C.     

激光喷丸强化对电化学充氢316L奥氏体不锈钢振动疲劳性能的影响

研究了激光喷丸强化对电化学充氢316L奥氏体不锈钢振动疲劳性能的影响.测试分析了不同激光功率密度喷丸316L不锈钢充氢试样的残余应力、显微硬度和微观组织结构,并对比研究其振动疲劳寿命和断口形貌.结果 显示,激光喷丸诱导材料表层位错密度增加,并有效细化晶粒,抑制了氢原子的入侵,同时复杂晶界和高密度位错增殖结构阻碍了氢原子...     

Effect of combined treatment on the stability of austenite and the mechanical properties of chromium-manganese steels

The phase composition, the stablity of austenite with respect to the γ→α′ transformation under a load, and the mechanical properties in torsion of Cr−Mn steels 17Kh13G7S, 20Kh14G7, and 40Kh14G7 with metastable austenite after straining and heat treatment by various regimes are investigated. Methods for a combined treatment of Cr−Mn steels are suggested in order to provide a high level of their strength and ductility properties. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 18–21, January, 1997.     

Fatigue life improvements of the AISI 304 stainless steel ground surfaces by wire brushing

The surface and subsurface integrity of metallic ground components is usually characterized by an induced tensile residual stress, which has a detrimental effect on the fatigue life of these components. In particular, it tends to accelerate the initiation and growth of the fatigue cracks. In this investigation, to deliberately generate compressive residual stresses into the ground surfaces of the AISI 304 stainless steel (SS), wire brushing was applied. It was found that under the experimental conditions selected in this investigation, while the surface roughness was slightly improved by the brushing process, the surface residual stress shifted from a tensile stress (σ_‖=+450 MPa) to a compressive stress (σ_‖=−435 MPa). On the other hand, the work-hardened deformation layer was almost two times deeper after wire brushing. Concerning the fatigue life, an improvement of 26% in terms of endurance limit at 2×10⁶ cycles was realized. Scanning electron microscope (SEM) observations of the fatigue fracture location and size were carried out to explain the fatigue life improvement. It was found that the enhancement of the fatigue strength could be correlated with the distribution and location of the fatigue fracture nucleation sites. Concerning the ground surfaces, it was seen that the fatigue cracks initiated at the bottom of the grinding grooves and were particularly long (150–200 μm). However, the fatigue cracks at the brushed surfaces were shorter (20–40 μm) and appeared to initiate sideways to the plowed material caused by the wire brushing. The results of the wire-brushed surface characterization have shown that significant advantages can be realized regarding surface integrity by the application of this low-cost process compared to shot peening.     

High resolution electron microscopy characterization of nitrides of iron and iron alloys

A study of nitride-phase formation in alloys nitrided by glow discharge plasma nitriding was performed by high-resolution electron microscopy. The iron and steel samples were nitrided above and below the eutectoid transformation point (590 ‡C). After nitriding, the samples were annealed for several treatment times at 400 ‡C. The microstructure and identification of the iron nitrided phases formed by nitriding and annealing were investigated by x-ray diffraction (XRD), optical microscopy (OM) and mainly high resolution electron microscopy (HREM) in a JEOL-4000EX high resolution microscope at 400 kV. The results of the characterization show a surface ε compact nitride layer, which is transformed into ε + γ during cooling. The tempering process affects the surface hardness of the samples. After an initial increase relative to the untreated sample, the microhardness diminishes after long treatment times. This behavior is correlated with the αt’’ phase appearance, growth, and transformation.     

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases     

Effect of Preliminary Heat Treatment on the Stability of Austenite in Steel 08Kh15N5D2T

The effect of the heating rate and long-term holds in the temperature range of inverse martensitic (α → γ) transformation on the stability of austenite in the cooling of chromium-nickel steel 08Kh15N5D2T is studied. Additional conditions for growth of stable austenite in the structure of the steel (to 60%) are determined. Isothermal diagrams of decomposition of retained austenite at negative temperatures after annealing in the (α + γ) range in different modes are plotted. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 17 – 19, June, 2005.