Effect of cryoforming of austenitic Cr Ni-steels at 77 K on martensitic transformation and work-hardening characteristics

The effect of cryoforming at 77 K on the flow and work-hardening characteristics was investigated considering the martensitic transformation behaviour in austenitic Cr Ni steels with different nickel contents. The test steels can be divided into two groups relating to the flow and work-hardening characteristics and martensitic transformation behaviour at 77 K. The first group comprises steels with less than 16 % nickel, the second group those with more than 16 % nickel. The flow curves of the first-group steels show two inflection points on the basis of γ → α'-transformation. α_γ'-martensite is observed and ?- and α_?'-martensite too. The flow curves of the second-group steels do not show any inflection points. The γ → α'-martensitic transformation is not induced, ?- and α_?'-martensite are provable by light and scanning microscopy. The stress-strain intervals were determined for the individual martensite transformations at 77 K in the test steels. They are dependent from the nickel content. The stress which specifies the first inflection point on the flow curve and the minimum of the work-hardening rate, respectively, characterizes the stress for initiating the deformation-induced α_γ'-martensite formation. Transformation of the austenite to α' martensite will end in achieving a stress of 1200 to 1400 MPa, i.e. in achieving the second inflection point of the flow curve and the maximum of the work-hardening rate, respectively. The stress interval is not dependent from the nickel content.     

Effect of Residual Stress and Strain-Induced α′-Martensite on Delayed Cracking of Metastable Austenitic Stainless Steels

The role of residual stresses and strain-induced α′-martensite in delayed cracking of metastable austenitic stainless steels was studied by means of Swift cup tests, measurement of residual stresses by X-ray diffraction and ring slitting, and α′-martensite content determination. Low-Ni, high-Mn austenitic stainless steels, e.g., AISI 201, were compared with Fe-Cr-Ni austenitic stainless steels. The presence of α′-martensite seemed to be a necessary prerequisite for delayed cracking to occur in austenitic stainless steels with typical internal hydrogen concentrations (<5 ppm). Stable low-Ni austenitic stainless steel was not prone to delayed cracking. The low-Ni metastable grades showed more severe cracking at lower degree of deformation and lower volume fraction of α′-martensite than that of the metastable 300-series grades. The limiting α′-martensite content, below which delayed cracking did not occur, decreased along with the nickel content of the material. The strain-induced martensitic transformation substantially increased the magnitude of residual stresses in deep-drawn cups. One explanation for high sensitivity of the low-Ni grades to delayed cracking after deep drawing is their higher residual stresses compared to that of the Fe-Cr-Ni grades. Alloying elements of the stainless steels, nickel, and carbon in particular, influence the sensitivity to delayed cracking through their effect on the properties of the α′-martensite.     

Stress Induce Martensitic Transformations in Hydrogen Embrittlement of Austenitic Stainless Steels

In austenitic type stainless steels, hydrogen concentration gradients formed during electrochemical charging and followed by hydrogen loss during aging, at room temperature, surface stresses, and martensitic phases α′-BCC and ε-HCP developed. The basic relationship between the X-ray diffraction peak broadening and the hydrogen gradients, formed during charging and aging at room temperature in such austenitic stainless steels, were analyzed. The results demonstrate that the impact of stresses must be considered in the discussion of phase transformations due to hydrogenation. Austenitic stainless steels based on iron-nickel-chromium, have relatively low stacking fault energy γ_SFE and undergo: quenching to low temperatures, plastic deformation, sensitization heat treatments, high pressure (≥3–5 × 10⁹ Pa) by hydrogen or other gases, electrochemical charging (when the sample is cathode) and when is irradiation by various ions the samples in vacuum. All the above mentioned induce formation of ε and α′ in the face-centered cubic (FCC) austenite γ matrix. The highest stresses cause formation of mainly α′ phase and ε-martensite, and both are involved in plastic deformation processes and promoting crack propagation at the surface. In 310 steel, the crack propagation is based on deformation processes following ε-martensitic formation only. Formations of ε- and α′-martensites were noted along the fracture surfaces and ahead of the crack tip. The cracks propagated through the ε-martensitic plates, which formed along the active slip planes, while α′ phase was always found in the high-stress region on the ends of the ligaments from both sides of the crack surfaces undergoing propagation.     

Effect of Cold Rolling on Microstructure and Mechanical Properties of AISI 301LN Metastable Austenitic Stainless Steels

 The microstructure and mechanical properties evolution of AISI 301LN metastable austenitic stainless steels during cold rolling were investigated. A wide range of cold thickness reduction (10%-80%) was carried out in a four-high rolling mill at ambient temperature. The X-ray and Feritscope MP30 were used to identify the strain-induced α′-martensite phase and its volume fraction, respectively. The microstructure was observed by optical micrograph and the mechanical properties were determined by tensile tests and microhardness. The results show that the strain-induced α′-martensite nucleated at the shear bands intersections and the growth of α′-martensite occurred by the repeated nucleation of new embryos. The volume fraction of strain-induced α′-martensite increased with increasing the cold rolling reduction. In addition, the percentage increased in the tensile strength is the same as that of hardness. The ratio between the average tensile strength and the average microhardness was found to range between 2.82 and 3.17.     

Dilatometric Study of Continuous Annealing of a Cold-Rolled Austenitic Stainless Steel

The sequence of structural changes produced in two deformed microstructures of austenitic stainless steel elaborated by a multipass unidirectional cold rolling (CR) and a two-step one, to a 75% thickness reduction, is followed by dilatometric experiments. The two materials show different dilatometric behaviors. X-ray diffraction and microhardness measurements are performed to underlie the observed dilatometric behaviors. The material subjected to multipass unidirectional CR shows an unusual dilatometric behavior. The first heating stage leads to the occurrence of the recovery reaction in competition with the ε-martensite reversion. When the temperature increases between 550 and 780 °C, the reversion of deformation-induced α-martensite takes place and leads to a complicated dilatometric anomaly. Further increase in temperature leads to the occurrence of the recrystallization transformation. However, the material subjected to two-step CR shows a quite usual dilatometric behavior which is explained by the occurrence of several reactions in the following order: 1) T < 300 °C, the recovery reaction, 2) 300 < T < 680 °C, the ferro- to paramagnetic transformation of α-martensite, the reversion of ε-martensite, and the athermal reversion of α-martensite, 3) 680 < T < 760 °C, thermal reversion of DIM, and then 4) at T > 760 °C, the recrystallization.     

Influence of strain-induced phase transformation on the surface crystallographic texture in cold-rolled-and-aged austenitic stainless steel

Stainless steels (SSs) having a stable and metastable austenitic phase were studied to see the influence of strain-induced phase transformation in the metastable austenitic stainless steel on the evolution of texture during cold rolling and aging. AISI 304L and 316L SS plates were unidirectionally cold rolled up to a 90 pct reduction and aged at different aging temperatures. The strain-induced transformation of austenite to α′-martensite phase and the evolution of texture in both the phases were studied as a function of rolling reduction as well as aging temperature in the metastable 304L austenitic stainless steel. The X-ray diffraction (XRD) technique was employed to quantify the volume fractions and characterize the texture of austenite and martensite phases in the rolled and aged conditions. Results are compared with the texture evolution in the stable austenitic 316L SS.     

Effect of Internal Hydrogen on Delayed Cracking of Metastable Low-Nickel Austenitic Stainless Steels

Metastable austenitic stainless steels, especially manganese-alloyed low-nickel grades, may be susceptible to delayed cracking after forming processes. Even a few wppm of hydrogen present in austenitic stainless steels as an inevitable impurity is sufficient to cause cracking if high enough fraction of strain-induced α′-martensite and high residual tensile stresses are present. The role of internal hydrogen content in delayed cracking of several metastable austenitic stainless steels having different alloying chemistries was investigated by means of Swift cup tests, both in as-supplied state and after annealing at 673 K (400 °C). Hydrogen content of the test materials in each state was analyzed with three different methods: inert gas fusion, thermal analysis, and thermal desorption spectroscopy. Internal hydrogen content in as-supplied state was higher in the studied manganese-alloyed low-nickel grades, which contributed to susceptibility of unstable grades to delayed cracking. Annealing of the stainless steels reduced their hydrogen content by 1 to 3 wppm and markedly lowered the risk of delayed cracking. Limiting drawing ratio was improved from 1.4 to 1.7 in grade 204Cu, from 1.7 to 2.0 in grade 201 and from 1.8 to 2.12 in grade 301. The threshold levels of α′-martensite and residual stress for delayed cracking at different hydrogen contents were defined for the test materials.     

Transformation characteristic of a Cr-Mn steel with austenite

The microstructures of three steels with about 0.35% C, 10% Mn and 13% Cr were investigated. After homogenization treatment, all steels have an austenitic microstructure with some carbide precipitates. During cooling to ?196°C as well as by plastic deformation ε-martensite and α-martensite are formed. The influence of the degree of deformation at different temperatures on microstructure and stability of austenite is described in detail. This information will be used to find a microstructure optimized to have a high cavitation resistance.     

Effect of strain-induced martensitic phase transformation on the formability of nitrogen-alloyed metastable austenitic stainless steels

Strain-induced martensitic phase transformation and its influence on the formability of newly developed nitrogen-alloyed metastable austenitic stainless steels were systematically investigated. Yield strength for the asreceived steels bearing lownickel content was around 300 MPa and their elongation ratios varied from 55. 2% to61. 7%. Erichsen numbers of these samples differed from 13. 82 to 14. 57 mm. Although its Cu content was lower than that of other samples,steel D2 exhibited better plasticity and formability,which was attributed to γ→α'martensitic phase transformation. EBSD,XRD,and magnetism tests showed that increases in deformation ratio gradually increased the α' martensite phase of a sample,thereby contributing to its strain and inducing the optimal transformation-induced plasticity effect. An M_(d30/50) temperature of around 20 ℃,which is close to the deformation temperature,provided the austenite with adequate stability and gradually transformed it into martensite,thereby endowing lean ASS with better formability.     

Tensile Deformation Behavior and Phase Transformation in the Weld Coarse-Grained Heat-Affected Zone of Metastable High-Nitrogen Fe-18Cr-10Mn-N Stainless Steel

The tensile deformation behavior and phase transformation in the weld coarse-grained heat-affected zone (CGHAZ) of a metastable high-nitrogen austenitic stainless steel was explored through tensile tests, nanoindentation experiments, and transmission electron microscopy analysis. True stress–strain response during tensile test was found to be seriously affected by δ-ferrite fraction, which depends on peak temperature of the CGHAZs. The strain-induced martensitic transformation (SIMT) occurred in base steel, whereas the SIMT disappeared and deformation twinning occurred predominantly in the CGHAZs. The relationship among true stress–strain response, nanoindentation hardness, and deformed microstructures was carefully investigated and discussed in terms of changes of stacking fault energy.     

Phase changes related to hydrogen-induced cracking in austenitic stainless steel

Hydrogen-induced phase transformation of the γ-f.c.c. phase to the ϵ-h.p.c. and α′ b.c.c. martensitic phases in austenitic stainless steel were investigated. By means of X-ray diffraction and transmission electron microscope (TEM) techniques, the austenitic stability of AISI type 316 stainless steel was studied over various aging times following 24 h of cathodic charging at room temperature. Splitting of the γ:γ1 and ϵ:ϵ1 diffraction peaks was observed. The α′-phase was formed in the early stage of aging. There was no evidence of an α′-phase shift during aging. The weight fraction of the α′-martensite increased from 7% in a coarse-grained (ASTM 2) specimen to 28% in a fine-grained (ASTM 11) type 316 steel. After cathodic charging with hydrogen, induced cracks propagated mainly along the {111}_γ planes. TEM observations of the surface cracking revealed that the cracking propagated mainly along the regions having a mixed structure of α′- and ϵ-martensitic phases.     

Metastable Phase Transformation in Ti-5Ta-2Nb Alloy and 304L Austenitic Stainless Steel under Explosive Cladding Conditions

Ti-5Ta-2Nb alloy was clad on 304L austenitic stainless steel (SS) using the explosive cladding process. Both Ti-5Ta-2Nb and 304L austenitic steel were severely deformed due to high pressure (in the gigapascal range) and strain rate (10⁵/s), which are characteristics of explosive loading conditions. Consequent changes produced in the microstructure and crystal structure of both the alloys are studied using electron microscopy techniques. The microstructure of both Ti-Ta-Nb alloy and 304L steel showed evidence for the passage of the shock waves in the form of a high number density of lattice defects such as dislocations and deformation twins. In addition, both the alloys showed signatures of phase transformation under nonequilibrium conditions resulting in metastable transformation products. 304L SS showed martensitic transformation to both ????(bcc) and ??(hcp) phases. Microscopic shear bands, shear band intersections, and twin boundaries were identified as nucleation sites for the formation of strain-induced phases. Ti-Ta-Nb alloy underwent metastable phase transformation to an fcc phase, which could be associated with regions having a specific morphology.     

In-Situ Measurements of Load Partitioning in a Metastable Austenitic Stainless Steel: Neutron and Magnetomechanical Measurements

In order to construct physically based models of the mechanical response of metastable austenitic steels, one must know the load partitioning between the austenite and the strain-induced martensitic phases. While diffraction-based techniques have become common for such measurements, they often require access to large facilities. In this work, we have explored a simple magnetic technique capable of providing a measure of the stresses in an embedded ferromagnetic phase. This technique makes use of the coupling between the elastic strain and the magnetic response of the $\alpha^{\prime}$ -martensite in an austenitic stainless steel undergoing straining. The magnetic technique proposed here is compared to neutron diffraction measurements made on the same material and is shown to give nearly identical results. The resulting predictions of the load partitioning to the $\alpha^{\prime}$ -martensite phase suggest that $\alpha^{\prime}$ deforms in a complex fashion, reflecting the fact that the microstructure is progressively transformed from austenite to martensite with straining. In particular, it is shown that the apparent hardening of the $\alpha^{\prime}$ -martensite suggests elastic deformation as an important source of high macroscopic work-hardening rate in this material.     

Effect of strain rate on the strain-induced <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis>′-martensite transformation and mechanical properties of austenitic stainless steels

The effect of strain rate on strain-induced γ → α′-martensite transformation and mechanical behavior of austenitic stainless steel grades EN 1.4318 (AISI 301LN) and EN 1.4301 (AISI 304) was studied at strain rates ranging between 3×10⁻⁴ and 200 s⁻¹. The most important effect of the strain rate was found to be the adiabatic heating that suppresses the strain-induced γ → α′ transformation. A correlation between the work-hardening rate and the rate of γ → α′ transformation was found. Therefore, the changes in the extent of the α′-martensite formation strongly affected the work-hardening rate and the ultimate tensile strength of the materials. Changes in the martensite formation and work-hardening rate affected also the ductility of the studied steels. Furthermore, it was shown that the square root of the α′-martensite fraction is a linear function of flow stress. This indicates that the formation of α′-martensite affects the stress by influencing the dislocation density of the austenite phase. Olson-Cohen analysis of the martensite measurement results did not indicate any effect of strain rate on shear band formation, which was contrary to the transmission electron microscopy (TEM) examinations. The β parameter decreased with increasing strain rate, which indicates a decrease in the chemical driving force of the α → α′ transformation.     

Deformation Induced Martensite Formation and its Effect on Transformation Induced Plasticity (TRIP)

The knowledge of the stress‐ and deformation‐induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress‐temperature‐transformation (STT) and a deformation‐temperature‐transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the M_d‐temperature for γ→?, ?→α', γ→?→α’ and γ→α’ martensite formation is defined by two stress‐temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformation‐induced martensite formation in the range of uniform elongation. The intersection point defines the corresponding M_d‐temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the M_d‐temperature. It defines the amount of martensite being formed. When the M_d^γ→? temperature is reached and the formation of the first deformation‐induced amount of ?‐martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the M_d^γ→α' or the M_d^γ→?→α' temperature ‐ similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation‐induced ?‐phase exists. The transformation plasticity in the test steel is considerably caused by the deformation‐induced ? and α’ martensite formation. Using the new evaluation method, the increase of plasticity ΔA (TRIP‐effect) and strength ΔR can be quantified.     

Mechanical Behavior of Deformation‐Induced α′‐Martensite and Flow Curve Modeling of a Cast CrMnNi TRIP‐Steel

For the modeling of the mechanical behavior of a two phase alloy with the rule of mixture (RM), the flow stress of both phases is needed. In order to obtain these information for the α′‐martensite in high alloyed TRIP‐steels, compression tests at cryogenic temperatures were performed to create a fully deformation‐induced martensitic microstructure. This martensitic material condition was subsequently tested under compressive loading at ?60, 20, and 100°C and at strain rates of 10^?3, 10⁰, and 10³ s^?1 to determine the mechanical properties. The α′‐martensite possesses high strength and surprisingly good ductility up to 60% of compressive strain. Using the flow stress behavior of the α′‐martensite and that of the stable austenitic steel AISI 316L, the flow stress behavior of the high alloyed CrMnNi TRIP‐steel is modeled successfully using a special RM proposed by Narutani et al.     

Effect of Primary Factor on Cavitation Resistance of Some Austenitic Metals

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Kinetics of strain-induced martensitic nucleation

Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of ε’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.     

Strain-Induced Phase Transformation and Nanocrystallization of 301 Metastable Stainless Steel Upon Ultrasonic Shot Peening

The following study investigated the strain-induced phase transformation in metastable austenitic 301 stainless steels via an ultrasonic shot peening treatment (USP) for 5 to 30 minutes. Following the USP, the microhardness increased to a depth of 400 μm and from 200 to 400 HV. The deformed grains and the phase transformation were monitored via X-ray diffraction and electron backscattered diffraction analysis. The grain evolution was studied via transmission electron microscopy. Approximately 500 nm α′-martensite grains formed in the top-most region after 5 minutes of the USP treatment. The grains were then further refined to ~?100 nm when the peening time increased to 10 and 15 minutes. The grains refined down to tens of nanometers after the specimen was treated for 30 minutes, where the phases were composed of α′-martensite (~?50 nm). There was a mixture of austenite with α′-martensite (~?25 nm). The grain refinement and the phase transformation of austenite to α′-martensite during ultrasonic shot peening were systematically investigated.