316L Material Research Articles

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.

Irradiation testing of 316L(N)-IG austenitic stainless steel for ITER

In the frame work of the European Fusion Technology Programme and the International Thermonuclear Experimental Reactor (ITER), ECN is investigating the irradiation behaviour of the structural materials for ITER. The main structural material for ITER is austenitic stainless steel Type 316L(N)-IG. The operating temperatures of (parts of) the components are envisaged to range between 350 and 700 K. A significant part of the dose–temperature domain of irradiation conditions relevant for ITER has already been explored, there is, however, very little data at about 600 K. Available data tend to indicate a maximum in the degradation of the mechanical properties after irradiation at this temperature, e.g. a minimum in ductility and a maximum of hardening. Therefore an irradiation program for plate material 316L(N)-IG, its Electron Beam (EB) weld and Tungsten Inert Gas (TIG) weld metal, and also including Hot Isostatically Pressed (HIP) 316L(N) powder and solid–solid joints, was set up in 1995. Irradiations have been carried out in the High Flux Reactor (HFR) in Petten at a temperature of 600 K, at dose levels from 1 to 10 dpa. The paper presents the currently available post-irradiation test results. Next to tensile and fracture toughness data on plate, EB and TIG welds, first results of powder HIP material are included.

Improving tensile strength and bend ductility of titanium/AlSI 304L stainless steel friction welds

The metallurgical and mechanical properties of friction welds between titanium and AISI 304L stainless steel were examined. Joint tensile strength increased when high friction pressure (>196 MN m−2) and high upsetting pressure (294 MN m−2) were used during welding. Although the surface roughness of the titanium substrate had no effect on joint strength, decreasing the surface roughness of the AISI 304L material did increase the tensile strength of completed joints. As welded dissimilar joints had poor bend test ductility and failed in the interface region. Detailed microscopy and X-ray diffraction analysis confirmed that the poor bend ductility was caused by a combination of high hardness of the titanium material immediately adjacent to the joint interface, the presence of unrelieved residual strain at the joint interface, and intermetallic phases formed during the welding operation. Detailed transmission electron microscopy and X-ray analysis confirmed that a thin layer rich in intermetallics was present in the as welded joints. (FeNiCr)Ti phases were formed during seizure formation and disruption; this provided the necessary conditions for anomalously high rates of diffusion of titanium in stainless steel, and of iron, chromium, and nickel in titanium. Low temperature post-weld heat treatment (PWHT), involving heating to 500–600°C followed by immediate air cooling, reduced intermetallic precipitation, promoted stress relaxation, and facilitated complete bonding across the whole joint interface. This treatment markedly improved bend ductility and had a negligible effect on joint tensile strength. High PWHT temperatures (≥900°C) and long holding times at temperature markedly reduced joint tensile strength and bend ductility, owing to excessive formation of intermetallic phases at the joint interface.MST/1521

Improving tensile strength and bend ductility of titanium/AlSI 304L stainless steel friction welds

AbstractThe metallurgical and mechanical properties of friction welds between titanium and AISI 304L stainless steel were examined. Joint tensile strength increased when high friction pressure (>196 MN m−2) and high upsetting pressure (294 MN m−2) were used during welding. Although the surface roughness of the titanium substrate had no effect on joint strength, decreasing the surface roughness of the AISI 304L material did increase the tensile strength of completed joints. As welded dissimilar joints had poor bend test ductility and failed in the interface region. Detailed microscopy and X-ray diffraction analysis confirmed that the poor bend ductility was caused by a combination of high hardness of the titanium material immediately adjacent to the joint interface, the presence of unrelieved residual strain at the joint interface, and intermetallic phases formed during the welding operation. Detailed transmission electron microscopy and X-ray analysis confirmed that a thin layer rich in intermetallics was...

Corrosion of welded austenitic stainless steels

The results of an investigation of austentic stainless steels fabricated in the United Kingdom are presented. All were welded with first choice filler materials. The weldment under test was representative of industrially fabricated assemblies. Even in the case of the 304L material, the filler was 308L. Results of testing show that welding degrades the corrosion resistance of all austentitic stainless steels investigated. In all materials, the weld bead microstructures were the most susceptible to corrosion. The HAZ microstructures were more corrosion susceptible than the wrought microstructure, but less so than the weld bead. Corrosion behaviour is dominated by the distribution of molybdenum across the y/8 interface. The authors feel that it is not possible to categorize corrosion behavior simply by the microstructural constituents because the formation mechanisms of the weld bead controls corrosion behavior.