Hot working and recrystallization of as-cast 317L

Stress-strain behavior and microstructure evolution during hot working of as-cast austenitic stainless steel alloy 317L is investigated by uniaxial compression of cylindrical specimens at a strain rate of 1 s⁻¹ over the temperature range 1000 °C to 1150 °C and up to a strain of one. The measured flow curves show little strain hardening, attributed in part to the high stacking fault energy (SFE) of the alloy. Dynamic recrystallization is not observed. Static recrystallization is observed to nucleate within the austenite matrix in the dendrite cores at dislocation microbands and in austenite immediately adjacent to a vermicular microconstituent, composed primarily of sigma and austenite and, occasionally, some delta ferrite. The recrystallization kinetics of 317L are retarded compared to as-cast 316L steel. The relatively sluggish recrystallization behavior is attributed in part to the higher SFE of 317L, which favors recovery over recrystallization, and in part to gradients in chemical composition and SFE, not found in 316L, in the dendritic microstructure. Thus, in the austenite near the interphase boundary, with high SFE, recovery initially replaced recrystallization, in contrast to recrystallization in the austenite more distant from the boundary. The recrystallization kinetics of both as-cast 317L and 316L were relatively slow compared to wrought stainless steels of comparative grain size and SFE, presumably due to the crystallographic texture and associated relatively low flow stress in the former materials. A kinetic model for recrystallization in as-cast 317L is developed and utilized to simulate evolution of the first cycle of recrystallization during various thermal-mechanical treatment schedules typically employed during primary breakdown of as-cast material.     

Evolution of Microstructure and Texture During Hot Compression of a Ni-Fe-Cr Superalloy

Superalloys are being employed in more extreme conditions requiring higher strength, which requires producers to forge products to finer grain sizes with less grain size variability. To assess grain size, crystallographic texture, and substructure as a function of forging conditions, frictionless uniaxial compression testing characteristic of hot working was performed on INCOLOY 945 (Special Metals Corporation, Huntington, WV), which is a newly developed hybrid of alloys 718 and 925, over a range of temperatures and strain rates. The microstructure and texture were investigated comprehensively using light optical microscopy, electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM) to provide detailed insight into microstructure evolution mechanisms. Dynamic recrystallization, nucleated by grain/twin boundary bulging with occasional subgrain rotation, was found to be a dominant mechanism for grain refinement in INCOLOY 945. At higher strain rates, static recrystallization occurred by grain boundary migration. During deformation, duplex slip along {111} planes occurred until a stable 〈110〉 fiber compression texture was established. Recrystallization textures were mostly random but shifted toward the compression texture with subsequent deformation. An exception occurred at 1423 K (1150 °C) and 0.001 seconds⁻¹, the condition with the largest fraction of recrystallized grains, where a 〈100〉 fiber texture developed, which may be indicative of preferential growth of specific grain orientations.     

Fracture and the formation of sigma phase,M23C6, and austenite from delta-ferrite in an AlSl 304L stainless steel

The decomposition of delta-ferrite and its effects on tensile properties and fracture of a hot-rolled AISI 304L stainless steel plate were studied. Magnetic response measurements of annealed specimens showed that the transformation rate of delta-ferrite was highest at 720 °C. Transformation behavior was characterized by light microscopy, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy on thin foils. The initial transformation of delta-ferrite (δ) to austenite (γ) and a chromium-rich carbide (M₂₃C₆) occurred by a lamellar eutectoid reaction, δ⇄M₂₃C₆ +γ. The extent of the reaction was limited by the low carbon content of the 304L plate, and the numerous, fine M₂₃C₆ particles of the eutectoid structure provide microvoid nucleation sites in tensile specimens annealed at 720 °C for short times. Sigma phase(σ) formed as a result of a second eutectoid reaction,δσ +γ. Brittle fracture associated with the plate-shaped sigma phase of the second eutectoid structure resulted in a significant decrease in reduction of area (RA) in the transverse tensile specimens. The RA for longitudinal specimens was not affected by the formation of sigma phase. Tensile strengths were little affected by delta-ferrite decomposition products in either longitudinal or transverse orientations. Y. Shen, formerly with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, is deceased.     

Quantitative measurement of deformation-induced martensite in 304 stainless steel by X-ray diffraction

A single X-ray diffraction scan is effectively used for identifying and evaluating deformation-induced transformation in 304 austenitic stainless steel. Variations in grain size influence surface constraint and hence the through-thickness transformation response. The initial stage of transformation in this steel is most likely dominated by –martensite formation.     

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10⁻⁴s⁻¹ to 400 s⁻¹. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.     

Effect of forging strain rate and deformation temperature on the mechanical properties of warm-worked 304L stainless steel

Stainless steel 304L forgings were produced with four different types of production forging equipment – hydraulic press, mechanical press, screw press, and high-energy rate forging (HERF). Each machine imparted a different nominal strain rate during the deformation. The final forgings were done at the warm working (low hot working) temperatures of 816 °C, 843 °C, and 871 °C. The objectives of the study were to characterize and understand the effect of industrial strain rates (i.e. processing equipment), and deformation temperature on the mechanical properties for the final component. Some of the components were produced with an anneal prior to the final forging while others were deformed without the anneal. The results indicate that lower strain rates produced lower strength and higher ductility components, but the lower strain rate processes were more sensitive to deformation temperature variation and resulted in more within-part property variation. The highest strain rate process, HERF, resulted in slightly lower yield strength due to internal heating. Lower processing temperatures increased strength, decreased ductility but decreased within-part property variation. The anneal prior to the final forging produced a decrease in strength, a small increase in ductility, and a small decrease of within-part property variation.     

Hot working and recrystallization of As-Cast 316L

Stress-strain behavior and microstructure evolution during hot working of as-cast austenitic stainless steel alloy 316L were investigated by uniaxial compression of cylindrical specimens at a strain rate of 1 s⁻¹ over the temperature range 1000 °C to 1150 °C and up to a strain of one. The measured flow curves showed monotonic hardening, indicating that dynamic recrystallization was not important in microstructural evolution. Static recrystallization was observed to nucleate preferentially at the delta ferrite-austenite interphase boundaries. The recrystallization kinetics of the as-cast material was compared to a relatively fine-grained wrought 316L material and found to be somewhat slower. However, the difference between the two material conditions was not nearly as great as previously reported for as-cast and wrought 304L alloy. The difference in behaviors between 316L and 304L is attributed to the relatively large amount and vermicular morphology of the delta ferrite phase in the 316L, resulting in a relatively fine effective grain size, compared to the existing coarse columnar structure, and concomitant enhancement of recrystallization. Compared to wrought 316L, the recrystallization rate of the as-cast material was relatively sluggish, despite a relatively fine effective grain size. The difference is associated with the 100 orientations of the columnar grains with respect to the compression axis, producing a soft orientation and a reduced rate of accumulation of dislocation density in the substructure. Also, compared to wrought 316L, the recrystallization rate of the as-cast material tends to decrease with time, the drop occurring concurrently with spheroidization and dissolution of the ferrite. It is suggested that (1) movement of the delta ferrite-austenite interphase boundary during spheroidization may poison incipient recrystallization and (2) dissolution of delta ferrite can locally enrich the austenite matrix in Mo and Cr, raising the local stacking fault energy and lowering grain boundary mobility to favor recovery over recrystallization in the vicinity of the ferrite-austenite boundary. A kinetic model for recrystallization was developed and used to simulate evolution of the first cycle of recrystallization during various thermal-mechanical treatment schedules typically employed during the primary breakdown of as-cast material.     

Sheet Formability and Performance of Metastable Austenitic Stainless Steels

The sheet formability of AISI Types 301, 304, and 305 stainless steels, ranked in order of increasing stability to strain‐induced martensite formation, was evaluated as a function of temperature between 15 and 60 °C. Forming limits spanning deep drawing, plane strain, and biaxial stretching strain states were determined by circle grid analysis of sheet specimens subjected to punch‐stretch testing at a constant punch displacement rate. Amounts of strain‐induced martensite were measured as a function of strain by magnetic measurement. Formability varied widely depending on test temperature and austenite stability, a result of temperature‐ and strain‐dependent formation of martensite that in some conditions was beneficial and in some conditions was detrimental to formability. These results are presented and discussed in detail.     

Controlled drawing to produce desirable hardness and microstructural gradients in alloy 302 wire

The production of a macroscopically duplex microstructure in stainless steel alloy 302 wire, fine grains on the wire surface and coarse grains at the wire interior, was investigated by systematically varying the drawing angle from 8 to 32 deg and the reduction from 1 to 15 pct. The measured hardness gradient was correlated to the microstructure after heat treating at 1000 °C for 0.5 hours. It was determined that the wire surface must exceed a hardness level of 207 KHN for recrystallization to a fine grain size, while the wire core must be hardened to a level between 166 and 207 KHN for grain growth. The deformation zone geometry parameter (Δ) for wire drawing, which is conventionally employed to give a relative measure of the strain distribution in a wire workpiece as a function of die angle and reduction, was utilized in the design of the experimental drawing schedules. The magnitude of measured hardness gradients and the corresponding calculated value of Δ were found to vary similarly with die angle but differently with reduction. At constant total reduction, multiple- and single-step drawing schedules produced equivalent hardness gradients, even though the calculated values for Δ indicated that the former would give a steeper gradient. Wires with two widely differing grain size gradients, coarse and fine vs. fine and coarse at the wire surface and center, were headed. The wire with fine grains on the surface had the higher resistance to surface cracking.     

Effect of hot working on structure and strength of type 304L austenitic stainless steel

The development of microstructure and strength during forging in a single-phase austenitic stainless steel, 304L, was investigated by means of forward extrusion of cylindrical specimens. The temperature, strain, and strain rate of deformation were varied. A low strain rate was imparted by press forging (PF), and a high strain rate by high-energy-rate forging (HERF). Low forging temperatures produced dynamically recovered microstructures and monotonic increases in strength with increasing strain for low and high strain rates. At higher forging temperatures, the high-energy-rate-forged material exhibited softening, after the application of a critical amount of strain, as a result of static recrystallization which occurred within a few seconds after cessation of deformation. Analysis of isothermal compression test data, specifically the strain-to-peak stress associated with the onset of dynamic recrystallization, confirmed that dynamic recrystallization would not be expected for the deformation conditions imposed during forward extrusion in this study. Recrystallized grain size was found to vary uniquely with strain, initial grain size, and the Zener-Hollomon parameter. Recrystallization was much less prevalent in press-forged material and may have been affected by die chilling as well as the predominance of dynamic recovery. The variation of strength, recrystallized grain size, and extent of recrystallization with the deformation parameters, temperature and strain, are presented as a set of processing-property maps for each forging technique (έ). The findings are discussed in the context of developing process design criteria for forging alloy 304L.