Effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel

The effects of sensitisation-induced martensitic transformation on the tensile behaviour of 304 austenitic stainless steel have been investigated. Yield strength is reduced by sensitisation, but ultimate tensile strength is nearly unaffected. Strain-hardening behaviour is changed by sensitisation, too. Although sensitisation may induce martensite formation near grain boundary, twin boundary, and austenite/martensite interface, the sensitisation-induced martensite does not exert a great influence on tensile behaviour in the 304 steel. In the unsensitised condition, martensitic transformation in the steel bulk induced by prior deformation and liquid-nitrogen immersion also does not change strain-hardening behaviour in the present steel.     

Aging of a copper bearing HSLA-100 steel

Investigations were carried out on aging of a HSLA-100 steel after varying amounts of cold deformation. Mechanical properties (hardness, tensile properties and toughness) were measured and structural changes were studied using optical, TEM and SEM techniques. As a result of various treatments, the hardness and UTS could be significantly improved, but with drastic fall in ductility and impact strength, especially in peak aged conditions. The parameters affecting impact strength were examined and it was concluded that various microstructural features affected toughness through their influence on tensile properties. In this steel the impact strength could be improved by lowering the UTS and increasing the ductility (pct elongation). The improvement in hardness and UTS was attributed to formation of thick precipitate-dislocation tangles. The aging process caused a slow transformation of lath martensite into acicular ferrite due to occurrence of in situ recrystallization. The concentration of Cu in particles precipitating on aging was followed using EDAX technique.     

Mechanical anisotropy in Ca-treated and ultra-low sulphur HSLA-100 steel

A few elongated MnS inclusions were observed in ultra-low sulphur HSLA-100 steel which significantly decreased the yield strength (YS), ultimate tensile strength and notch toughness of short transverse direction. The slight anisotropies of YS (≈5?MPa) and notch toughness (≈15?J) between longitudinal and transverse directions are correlated to the presence of texture components in the vicinity of {113}<110> and {554}<225>. In the case of anisotropy between longitudinal and short transverse directions, it is found that recognising the contribution of each effective factor in the creation of strength anisotropy is not possible. However, the most primary effective factor in the creation of remarkable notch toughness anisotropy (≈70?Pct), especially at ?85°C, is related to the existence of elongated MnS inclusions.     

Weldability and properties of martensitic/austenitic stainless steel joints

Abstract

A series of studies has been carried out to examine the weldability and properties of dissimilar steel joints using martensitic and austenitic stainless steels F6NM (OCr13Ni4Mo) and AISI 347, respectively. This type of joint requires good mechanical properties, corrosion resistance, and a stable magnetic permeability in addition to a good weldability. Weldability tests include weld thermal simulation of the martensitic steel to investigate the influence of weld thermal cycles and post-weld heat treatment (PWHT) on the microstructure and mechanical properties of the heat affected zone (HAZ); implant testing to examine the tendency for cold cracking of martensitic steel; and rigid restraint testing to determine hot crack susceptibility of the multipass dissimilar steel joints. The simulation results indicated that the toughness of the martensitic steel HAZ did not change significantly after the weld thermal cycles. The implant test results indicated that welds produced using nickel based filler show no tendency for cold cracking, whereas welds produced using martensitic or ferritic filler show such a tendency. Based on the weldability tests, a welding procedure (tungsten inert gas welding for root passes with HNiCrMo-2B wire followed by manual metal arc welding using ENiCrFe-3B coated electrode) was developed and a PWHT at 600°C for 2 h was recommended. Joints produced using the developed welding procedure are not susceptible to hot and cold cracking. After PWHT the joints exhibit both satisfactory mechanical properties and stress corrosion cracking resistance.

MST/1955     

Tribological behavior of NiTi alloy in martensitic and austenitic states

The wear behavior of Ti–50.3 at% Ni alloy in martensitic and austenitic states was studied. The alloy was prepared in a Vacuum Induction Melting furnace, forged at 800 °C, annealed at 1000 °C for 12 h, quenched in water, then aged at 400 °C for 1 h and followed by water quenching. The phase transformation temperatures were measured by differential scanning calorimetry. The shape memory and pseudoelasticity properties of NiTi were obtained by three-point bending test. The highest deflection recovery due to the pseudoelasticity was observed at temperature of 50 °C. The wear tests were conducted using a pin-on-disk tribometer in a water media at temperatures ranging from 0 °C to 50 °C. The results showed that the wear rate of NiTi alloy was decreased as the wear testing temperature increased. This was mainly attributed to the pseudoelasticity effect and higher strength of the alloy in the austenitic state at temperature of 50 °C. The results also showed a lower coefficient of friction in the austenitic state compared to the martensitic state.     

Characterization of strain-induced martensitic transformation in a metastable austenitic stainless steel

Kinetics of deformation-induced martensitic transformation in metastable austenitic steel AISI 301 was characterized by several techniques including classical light metallography, scanning electron microscopy, X-ray diffraction, neutron diffraction and electron back scattered diffraction. In situ monitoring of magnetic properties, acoustic emission and temperature increase during tensile tests at different strain rates was also performed. Results obtained by different methods are compared and discussed.     

Effect of aging on fatigue crack growth behaviour of Cu bearing HSLA-100 steel

Abstract

The effect of aging on fatigue crack growth rate (FCGR) of Cu bearing HSLA-100 steel has been studied. The steel was solution treated, water quenched and aged at various temperatures in the range of 350–700°C. The fatigue crack growth resistance of the steel decreased for the initial stages of aging from 350–500°C. Further aging up to 650°C resulted in an improvement in the crack growth resistance. Beyond 650°C, once again an inferior crack growth resistance was observed. This nature of variation of FCGR behaviour was similar to the trend portrayed by the strength properties with aging treatment. The results are related to the changes in the microstructural constituents owing to the aging treatment.     

Effect of microalloying on aging of a Cu-bearing HSLA-100 (GPT) steel

Investigations were carried out on aging of a HSLA-100 steel containing Cu as the major alloying element and Nb, Ti and V as microalloying elements. The aging process after varying amounts of cold deformation was followed by hardness measurements and microstructural changes were studied using light and electron microscopy. Presence of Ti activates the formation of (Nb, Ti)C precipitates and completely suppresses the precipitation of Cu. Even a solution treatment at 1100°C is not sufficient to completely dissolve Nb and Ti in the matrix and undissolved (Nb, Ti)C precipitates were observed in oil quenched state. Strain induced aging at 400° C causes simultaneous coarsening of existing precipitates and nucleation of fresh carbides, which results in multi-stage hardening in this steel. Strong precipitate-dislocation interactions cause retardation in recrystallization of deformation structure leading to retention of high hardness levels even on prolonged aging     

Magnetic measurements of martensitic transformation in austenitic stainless steel after room temperature rolling

In order to investigate the detection of martensite phase in deformed austenitic stainless steel, magnetic properties were examined by means of super conducting quantum interface device (SQUID) and vibrating sample magnetometer (VSM) techniques. Stainless steel specimens were rolled at room temperature with 15 to 55% reduction in thickness. Results indicate that the magnetic properties of stainless steel were sensitive to percent reduction in thickness and micro structural condition of stainless steel. It was found that saturation magnetization, amount of martensite and hardness increased whereas, coercive force and remanence ratio decreased with increasing percent reduction in thickness. The saturation magnetization depends mainly on amount of martensite, while the coercive force and remanence depends mostly on shape and distribution of martensite phase.     

Irradiation behavior of nanostructured 316 austenitic stainless steel

In order to get information about radiation resistance of ultrafine grained austenitic stainless steels, a 316 steel was deformed by high pressure torsion. The mean diameter of the grain after deformation was 40 nm. This material was annealed at 350 °C for 24 h or irradiated with 160 keV iron ions at 350 °C. Changes in the microstructure during annealing or irradiation were characterised by transmission electron microscopy (grain size) and laser assisted tomographic atom probe (solute distribution). Results indicate that this annealing has no influence on the grain size whereas the grain diameter increases under irradiation. Concerning the solute distribution, atom probe investigations show evidence of radiation-induced segregation at grain boundaries. Indeed, after irradiation, grain boundaries are enriched in nickel and silicon and depleted in chromium. On the contrary, no intragranular extended defects or precipitation are observed after irradiation.     

The application of metallographic techniques to the study of the tempering of HSLA-100 steel

A study of a quenched, and a quenched and tempered low-carbon, copper-containing steel (HSLA 100) was conducted. The primary investigative technique employed was transmission electron microscopy. Quenching the steel from the austenitizing temperature yielded a microstructure that was predominantly lath martensite. However, a significant amount of retained austenite was also present. Niobium carbide particles were also documented in the as-quenched structures. Tempering at 605°C for 1–3 h yielded a heterogeneous distribution of ---Cu, much of which was associated with the lath boundaries. The austenite was highly resistant to decomposition during tempering at 605°C and, as a consequence, little evidence of cementite precipitation was found.     

Effect of cold work and aging on mechanical properties of a copper bearing HSLA-100 steel

Influence of cold working and aging on the mechanical properties of a Cu-bearing HSLA-100 steel has been studied. The steel was given solution treatment at 1000°C, followed by cold rolling to 25, 50 and 80 pct deformations and aging at 600°C for various durations. Substantial improvement in hardness and UTS was observed in the peak aged condition of various treatments, but at the expense of ductility and impact-energy. Extensive scanning electron microscopic studies carried out on impact and tensile fracture surfaces suggest that poor impact energy and low ductility in peak aged condition could be associated with inhomogeneous deformation caused by the existence of coherent precipitates. Other parameters adversely affecting toughness and ductility in various stages of cold work and aging may include high stress concentration at high density dislocation network and dislocation-precipitate interface. Overaging in various treatments resulted in higher impact energy and ductility, presumably due to existence of incoherent precipitates and reduction in stress concentration at dislocation cell boundaries. It is observed that a good combination of high hardness and UTS with high impact energy and ductility could be obtained by a treatment suitable to cause coexistence of coherent and incoherent precipitates.     

Characterization of cyclic plastic bending of austenitic AISI 304 stainless steel

Steels used in chemical, nuclear and power industries are usually subjected to repeated loads that may induce significant inelastic deformations. Successful assessment of a component life places a great demand on understanding of material behaviour under strain histories involving cyclic plasticity under bending conditions.

This work is concerned with an experimental study of low-cycle fatigue behaviour under both load and deflection controlled conditions. The tests were conducted on AISI 304 stainless steels with the objective of determining strain cycling properties under cyclic plastic bending. These properties are characterized by the cyclic stress-strain hysteresis responses obtained.

The cyclic bending plasticity behaviour is investigated for beams with circular cross-section at two conditions of the material, under constant strain and under constant stress. Hysteresis loops for five different strain ranges of ±0.4%, ±0.82%, ±1.1%, ±1.33% and ±1.55% were obtained and also for different stress ranges of 730, 800, 870 and 940 MPa. The material exhibits cyclic hardening in its virgin state, the rate of hardening is very rapid at first but diminishes quickly as the stabilized condition is approached. The cyclic hardening for the constant strain condition is higher than the constant load condition.

The results also confirm the applicability of the reference stress method for predicting the cyclic moment-curvature curve for beams with circular cross-sections. The observed strain values are higher in the lower strain range and lower in the higher strain. However, the correlation obtained is considered quite satisfactory in view of the nonlinearity of the problem.     

Cyclic strain rate effect on martensitic transformation and fatigue behaviour of an austenitic stainless steel

In this study, the effect of strain rate on the cyclic behaviour of 304L stainless steel is investigated to unveil the complex interrelationship between martensitic phase transformation, secondary hardening, cyclic deformation and fatigue behaviour of this alloy. A series of uniaxial strain controlled fatigue tests with varying cyclic strain rates were conducted at zero and non‐zero mean strain conditions. Secondary hardening was found to be closely related to the volume fraction of strain‐induced martensite which was affected by adiabatic heating due to increasing cyclic strain rates. Tests with lower secondary hardening rates maintained lower stress amplitudes during cyclic loading which resulted in longer fatigue lives for similar strain amplitudes. Fatigue resistance of 304L stainless steel was found to be more sensitive to changes in strain rate than the presence of mean strain. The mean strain effect was minimal due to the significant mean stress relaxation in this material.     

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb-Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.     

Effect of plasma ion implantation on the hydrogen embrittlement of Cu strengthened HSLA-100 steel

The effect of low dosage plasma ion implantation on hydrogen embrittlement was studied for an HSLA steel using notched tensile samples. The plasma treatment caused an enhancement in the linear strain to failure under embrittling conditions. This was however not reflected in the fracture surfaces of the treated samples which had similar fractographic features as those of untreated samples. The plasma treatment delayed the process of embrittlement without causing any alteration in the basic mechanism of embrittlement. This was due to introduction of residual compressive stresses as well as reduction in the hydrogen permeation flux. Implantation in pure nitrogen seemed most beneficial while implantation in pure argon caused very little improvement.