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.     

The effect of grain size on the shock-loading response of 304-type stainless steel

The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes-23, 55, 85 and 187 Μm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both e and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pct cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.     

Martensite Formation in Hydrogen-Containing Metastable Austenitic Stainless Steel During Micro-Tension Testing

Deformation-induced martensite in type 304 stainless steel during micro-tension testing was characterized. The stress-strain behavior of uncharged and hydrogen-charged specimens revealed that hydrogen hastened the onset of hardening but decreased the strain-hardening rate, leading to premature plastic instability. In both specimens, a set of twin-related α′-martensite variants with Kurdjumov–Sachs relationships was prevalent. The Nishiyama–Wasserman relationship variant was also formed, but it was suppressed by hydrogen. This may be attributed to differences in the underlying deformation microstructure.     

Effect of High-Pressure Torsion Processing and Annealing on Hydrogen Embrittlement of Type 304 Metastable Austenitic Stainless Steel

The effect of high-pressure torsion (HPT) and annealing on hydrogen embrittlement (HE) of a type 304 stainless steel was studied by metallographic characterization and tensile test after hydrogen gas charging. A volume fraction of ~78 pct of the austenite transformed to α′ martensite by the HPT processing at an equivalent strain of ~30. Annealing the HPT-processed specimen at a temperature of 873 K (600 °C) for 0.5 hours decreased the α′ martensite to ~31 pct with the average grain size reduced to ~0.43 μm through the reverse austenitic transformation. Hydrogen charge into the HPT-processed and the HPT+annealed specimens in the hydrogen content of ~10 to 20 ppm led to no severe HE but appeared in the solution-treated specimen. Especially the 873 K (600 °C) annealed specimen had the ~1.4 GPa tensile strength and the ~50 pct reduction of area (RA) despite the hydrogenation.     

Electron backscattered diffraction study of <Emphasis Type="Italic">ε/α′</Emphasis> martensitic variants induced by plastic deformation in 304 stainless steel

The electron backscattered diffraction (EBSD) technique has been used to assess crystallographic features of the residual γ phase and the strain-induced ε/α′ martensites in a 304 stainless steel, tensile tested to 10 pct strain at T=−60 °C. The martensitic transformation rate varies according to the γ-grain orientation against the applied stress and the γ-grain size. The α′-transformation textures as well as the γ-misorientation spreads observed in specific γ-grain orientations have been analyzed. Large misorientation spreads are observed in the less-transformed γ grains. This reveals an important crystallographic slip activity, even if less strain-induced martensite has been formed. A strong γ → α′ variant selection was detected in the cube- and Goss-oriented γ grains for which the transformation is less developed. For the {110} 〈1–11〉 and copper-oriented γ grains, the amount of α′ martensite is significantly higher and the γ → α′ variant selection is less pronounced. This variant selection is then analyzed on at a local scale and is related to the presence of {111} _γ localized deformation bands on which further ε/α′ martensites have nucleated.     

The effect of grain size on the shock-loading response of 304-type stainless steel

The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes—23, 55, 85 and 187 μm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both ɛ and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pet cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.     

Comparative study of the impact response and microstructure of 304L stainless steel with and without prestrain

This study compares the dynamic plastic deformation behavior and microstructural evolution of 304L stainless steel with and without metal-forming prestrain, using the compressive split Hopkinson pressure-bar technique and transmission electron microscopy (TEM) under strain rates ranging from 8 × 10² to 5 × 10³ s⁻¹ at room temperature, with true strains varying from yield to 0.3. Results show that the flow stress of unprestrained and prestrained 304L stainless steel is sensitive to applied strain rate, but the prestrained material exhibits greater strength. A higher work-hardening rate and higher strain-rate sensitivity are also found in the prestrained material, while an inverse tendency exists for the activation volume. A constitutive equation with our experimentally determined specific material parameters successfully describes both unprestrained and prestrained dynamic behavior. Microstructural observations reveal that the morphologies of dislocation substructure, mechanical twins, microshear bands, and α′ martensite formation are strongly influenced by prestrain, strain, and strain rate. The density of dislocations increases with increasing strain and strain rate for both materials. The dislocation cell size decreases with increasing strain, strain rate, and prestrain. An elongated cell structure appears in the prestrained material as heavy deformation is applied. Mechanical twins are found only in the prestrained material. Microshear bands and α′ martensite are more evident at large strains and strain rates, especially for the prestrained material. Quantitative analysis indicates that the amount of dislocations, mechanical twins, and α′ martensite varies as a function of work-hardening stress (σ−σ _y), reflecting different strengthening effects and degrees of microhardness.     

The Relationship of Microstructure and Texture to Rolling Direction in Cold-Rolled Metastable Austenitic Steel

The microstructures and textures of strain-induced martensite and deformed austenite contained within metastable austenitic steel were investigated under unidirectional rolling and cross-rolling conditions. Under unidirectional rolling, the laths of α′ martensite that were produced were uniformly thinner with increasing thickness reduction. On the other hand, under cross-rolling, the resulting martensite laths varied in width, were irregularly bent, or were changed into polygonal cells. Deformed austenite possessed a similar texture showing an α _fcc-fiber regardless of the rolling conditions. However, for α′ martensite, the {001}〈110〉 _α orientation was strongly developed in cross-rolled specimens, while {115}〈110〉 _α and {335}〈110〉 _α evolved under unidirectional rolling.     

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

Plane-strain compression testing was carried out above, around, and below the A _r3 temperature with the deformation temperature, T _def, varying between 1323 K and 973 K (1050 °C and 700 °C), using Gleeble 3500, to develop uniform distribution of ultra-fine ferrite (UFF) grains. Prior austenite (γ) grain structure, developed after soaking at 1473 K (1200 °C), was mixed in nature, comprising both coarse- and fine-γ-grain sizes. Applying heavy deformation in a single pass, just above the austenite-to-ferrite (α) transformation temperature (A _r3), and cooling to room temperature resulted in the formation of UFF grain sizes (average α-grain size ~2 to 3 μm), with the largest grain sizes extending up to ~10 to 12 μm. Water quenching just after deformation prevented the coarsening of UFF grains and restricted the largest grain sizes to under 6 μm. Although the ferrite grain structures appeared homogeneous in slowly cooled samples (cooling rate (CR) 1 K/s), careful observation revealed the presence of alternate bands of coarse- (5 to 10 μm) and fine-α grains (<1 to 3 μm). The final α-grain size distributions were explained in view of the starting γ-grain size variation, dynamic recrystallization (DRX) of γ, dynamic strain-induced γ-to-α transformation (DSIT), and DRX of α and grain growth during slow cooling. Electron backscattered diffraction analysis (EBSD) revealed the presence of a large fraction (70 to 80 pct) of high-angle boundaries, having misorientation ≥15 deg. Compared to the use of the single, heavy deformation pass, the application of a number of lighter passes between A _e3 and A _r3 temperatures is more suitable in industrial rolling conditions, and also has the potential of developing UFF grains with high-angle boundaries.     

Microstructure and martensitic transformations in a dual-phase α/β Cu−Zn alloy

The martensitic transformations in a dual-phase α/β Cu−Zn shape-memory alloy, containing 15 pct by volume of α particles, were studied during subcooling and deformation. The crystal structure and characteristics of the martensitic transformation of a dual-phase Cu−Zn alloy were found to be similar to those of a single-phase alloy. Both the thermal martensite formed by subcooling and the stress-induced martensite (SIM) formed by loading possessed an M9R long-period stacking-order (LPSO) structure, with internal stacking faults on the (001) basal plane. Upon subcooling, the α particles were deformed in order to accommodate the shape strain accompanying the martensitic transformation. Although most of them are deformed by slip, deformation twins have, nevertheless, been found in a few α particles. Upon loading, the SIM with an M9R structure nucleates and grows at a given temperature; subsequently, another martensite phase (α_S) possessing an fct structure is formed, with a shear developing on the basal plane of the initial M9R SIM during further loading. However, during unloading, both the α_S and SIM are transformed and follow the reverse sequence back to the parent phase. However, some residual SIM and α_S were found at zero load, due to a constraint effect of the deformed α particles and grain boundaries. The α_S martensite may be formed by two intersecting plates of SIM or by advanced deformation on a single plate of SIM. In addition to the residual SIM and α_S martensite, an α_S lamellar martensite was found in the deformed specimen.     

The influence of austenite stability on the hydrogen embrittlement and stress- corrosion cracking of stainless steel

A study has been made of the HE and SCC of a type 304 and a type 310 austenitic stainless steel, and the results correlated with the presence or absence of α′ martensite, determined by means of a ferrite detector. Hydrogen induced slow crack growth (SCG) was observed at room temperature when type 304 was stressed i) in 1 psig (∼10⁵ N/m²) gaseous hydrogen, ii) after high temperature charging, and iii) while undergoing cathodic charging. The fracture surfaces corresponding to SCG were primarily transgranular and cleavage-like, and were found to be associated with α′. Conditions i) to iii) did not produce SCG in the type 310 steel, in which α′ martensite was not detected, nor did SCG occur when type 304 was stressed in gaseous hydrogen above the M_D temperature (∼110°C). These observations indicated that the formation of the martensitic phase was a prerequisite for SCG under these test conditions. Stressing of type 310 while it was undergoing cathodic charging at room temperature was found to produce shallow, nonpropagating cracks, confirming earlier reports that austenite can be embrittled by hydrogen in the absence of α′. SCC occurred in both alloys in boiling aqueous MgCl₂ (154°C) with no evidence for α′ formation. The results are discussed in terms of the mechanisms of HE and SCC. Formerly Research Associate, Department of Metallurgy and Mining Engineering, University of Illinois. Formerly Corrosion-Control Analyst with the Physical Plant at the University of Illinois.     

Grain refinement in 7075 aluminum by thermomechanical processing

A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.     

Martensitic Transformation During Cold Rolling Deformation of an Austenitic Fe-26Mn-0.14C Alloy

A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), α′-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to ε-martensite. However, a small fraction (~4 pct) of α′-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of ε-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m² because of the increase in temperature during the cold rolling deformation.     

The effects of strain rate and welding current mode on the dynamic impact behavior of plasma-arc-welded 304L stainless steel weldments

This article uses a split-Hopkinson pressure bar to investigate the effects of strain rate in the range of 10³ s⁻¹ to 8 × 10³ s⁻¹ and welding current mode upon the dynamic impact behavior of plasma-arc-welded (PAW) 304L stainless steel (SS) weldments. Stress-strain curves are plotted for different strain rates and welding parameters, and optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques are used to analyze the microstructure and fracture characteristics of the weldments. The results confirm that the strain rate and the welding current mode have a significant influence upon the dynamic impact behavior and microstructure evolution of 304L SS weldments. It is shown that for a constant strain, the flow stress increases with strain rate for both welding current modes, and that the pulsed current (PC) mode results in a higher weldment strength than the continuous current (CC) mode. Weldments fabricated using the PC mode exhibit an improved resistance to thermal softening, a greater strain-rate sensitivity, and a lower activation volume. The OM and SEM observations reveals that an adiabatic shear band dominates the fracture characteristics of both weldment types under impact loading. Microstructural analysis reveals that for both welding current modes, the dislocation density and volume fraction of α′ martensite increase with an increasing strain rate, while the twin formations reduce under the same conditions. Comparing the evolution of the microstructure in the base metal and the fusion zone, it is found that for both welding current modes, a higher dislocation density exists in the fusion zone, and that a larger volume fraction of α′ martensite and a greater twin density are present in the base metal. Furthermore, the dislocation density and volume fraction of α′ martensite is greater in PC weldments than in their CC counterparts. Finally, the present results indicate that the PC welding mode produces a weldment with superior dynamic impact response and improved weldment fracture characteristics.     

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.     

Young’s modulus and mechanical properties of Ti-29Nb-13Ta-4.6Zr in relation to α″ martensite

An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (M _s) temperature. A critical grain size of 40 μm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low M _S temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy.     

Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels

An investigation of phase transformations in hydrogen-charged metastable austenitic stainless steels was carried out. Solution-annealed, high-purity, ultralow-carbon Fel8Crl2Ni (305) and laboratory-heat Fel8Cr9Ni (304) stainless steels were examined. The steels were cathodically charged with hydrogen at 1, 10, and 100 mA/cm², at room temperature for 5 minutes to 32 hours, in an lN H₂SO₄ solution with 0.25 g/L of NaAsO₂ added as a hydrogen recombination poison. Changes in microstructure and hydrogen damage that resulted from charging and subsequent room-temperature aging were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Hydrides from hydrogen charging (hep ε* in 305 SS and fcc γ* and hcp ε* in 304 SS) were observed. The evidence suggests the following mechanisms for hydride formation during charging: (1)γ → ε → ε* hydride and (2) γ → γ* hydride. These hydrides were found to be unstable and decomposed during room-temperature aging in air by the following suggested mechanisms: (1)ε* hydride (hcp) → expanded ε (hcp) phase →α′ (bcc) phase and (2) γ* hydride →γ phase. The transformation from ε* toα′, however, was incomplete, and a substantial fraction of ε was retained. A kinetics model for hydride decomposition and the accompanying phase transformation during aging is proposed.     

The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution

A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill. The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening. The Institut de Recherches de la Sidérurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature. Formerly Graduate Student, The Centre for Metallurgical Process Engineering, The University of British Columbia Metallurgical transactions a