Showing papers on "Tempering published in 2010"

Thermal stability of austenite retained in bainitic steels

Abstract: Steels with a microstructure consisting of a mixture of bainitic ferrite and carbon-enriched retained austenite are of interest in a variety of commercial applications because they have been shown to exhibit good combinations of strength, toughness and ductility. However, their use at temperatures moderately above ambient requires a knowledge of the thermal stability of the austenite. The changes that occur during the tempering of a mixture of bainitic ferrite, carbon-enriched retained austenite and martensite have been characterised. An analysis of the volume change due to transformation shows that it is possible to distinguish the decomposition of austenite from the tempering of martensite. The nature of the carbides that form during the heat treatment is discussed as are the implications on the development of mathematical models accounting for calculating the strain during austenite decomposition and martensite tempering. It is found that the early stages of tempering reactions where the austenite content is not greatly reduced, can dramatically influence the stability of the austenite as it is cooled to ambient temperature.

Effect of vanadium and titanium modification on the microstructure and mechanical properties of a microalloyed HSLA steel

Abstract: DMR-249A is a low carbon microalloyed high-strength low-alloy (HSLA) steel. While DMR-249A plates of thickness less than 18 mm meet the specified room temperature yield strength (390 MPa) and Charpy impact toughness (78J at −60 °C) in the as-rolled condition, thicker plates require water quenching and tempering. Elimination of the quenching and tempering treatment can result in significant cost and energy savings besides offering increased productivity. Therefore, in the present work, modifications to the base DMR-249A steel composition have been investigated with the objective of producing thicker gage plates (24 mm) capable of meeting the specified properties in the normalized condition. Plates from three modified compositions i.e., containing 0.015 wt.% titanium and 0.06, 0.09 and 0.12 wt.% vanadium respectively and one composition with 0.10 wt.% vanadium, and without any titanium were investigated over a range of normalizing temperatures (875–1000 °C). In all cases, only the steel without titanium met the specified properties in the normalized condition. Microstructural investigations using scanning and transmission electron microscopy, as well as support evidence from calculations performed using ThermoCalc software, suggest that this is due to the presence of nanoscale vanadium rich carbonitride particles distributed throughout the matrix. These particles were absent in the titanium-containing steel at a similar vanadium level.

Microstructural evolution and low temperature impact toughness of a Fe-13%Cr-4%Ni-Mo martensitic stainless steel

Abstract: The microstructural evolution of a low carbon Fe-13%Cr-4%Ni-Mo (wt.%) martensitic stainless steel has been studied using transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP). The as-quenched sample has a typical lath martensite structure, but carbon atoms were found to form ultra-fine clusters, or carbon-enriched regions, in the martensite. After a single-stage tempering at 680 degrees C. the steel mainly consists of the martensite and a certain amount of fine M(23)C(6) carbides, which act as heterogeneous nucleation sites of the reversed austenite formed in the two-stage tempered (680 degrees C x 4 h + 600 degrees C x 4 h) samples. 3DAP investigations have revealed that the Ni, Cr and Mn atoms are enriched in the reversed austenite as contrasted with the martensite. Phosphorus atoms were uniformly distributed only in martensite. carbon was not detected in either martensite or austenite, and there was no segregation at the martensite-austenite interfaces. Those partitioning behaviors of the alloying elements were thought to be a factor contributing to the stability of the reversed austenite. High impact toughness at low temperature was obtained due to the existence of the reversed austenite. (c) 2009 Published by Elsevier B.V.

A study on heat affected zone softening in resistance spot welded dual phase steel by nanoindentation

Abstract: The base metal (BM) and the heat affected zone (HAZ) of a resistance spot welded dual phase steel have been evaluated by nanoindentation hardness testing. Three different surface conditions have been explored on the BM for assessing the nanohardness response. Softening has been investigated along the sub-critical HAZ by making nanoindentations on individual phases such as ferrite and tempered martensite (TM) at various distances from the line of lower critical temperature Ac1. A broken appearance accompanied with sub-micron particles were consistently found on TM at 100 μm from the Ac1 line suggesting carbide precipitation along with partial recovery of martensite. The morphology of TM kept on changing while moving away from Ac1 towards the BM as the fraction of broken appearance was reduced and the sub-micron particles became finer. SEM observations resulted in good agreement with the nanohardness of the TM phase along the sub-critical HAZ. In contrast, microhardness results suggested the termination of tempering at a shorter distance with respect to Ac1 and hence a reduced extension of the softening region. The improved resolution for assessing softening through nanoindentation was due to the possibility of avoiding the contribution of the phase boundaries because of the smaller size of the indentation; this also permitted evaluation of TM at low peak temperatures far from Ac1 where early stages of tempering took place.

Effect of heat treatment on microstructure and mechanical properties of high boron white cast iron

Abstract: The effect of different tempering temperatures on the microstructure and mechanical properties of high boron white cast iron after air quenching was studied. The experimental results indicate that the high boron white cast iron comprises a dendritic matrix and interdendritic boride M 2 B that distributed in the form of continuous network and the matrix is composed of fine pearlite at as-cast condition. After austenitising at 920 °C and air quenching, the matrix of high boron cast iron is changed from pearlite to lath-type martensite, and the morphology of boride is still kept in the form of continuous network. After tempered at 200 °C or a higher temperature, a secondary precipitation with a size of tens of nanometers is found in the matrix, and the size and the amount of this secondary precipitation increase with the increasing of tempering temperature. TEM analysis shows that the secondary precipitates have a crystal structure of M 23 (B,C) 6 . With the increasing of tempering temperature after air quenching, the hardness decreases, and the impact toughness firstly increases to a maximum value and then decreases. The optimum structure that has a combination of high hardness and high impact toughness can be obtained when the tempering temperature is lower than 400 °C.

Influence of Tempering Process on Mechanical Properties of 00Cr13Ni4Mo Supermartensitic Stainless Steel

Abstract: To investigate the influence of tempering process on microstructural evolutions and mechanical properties of 00Cr13Ni4Mo supermartensitic stainless steel (SMSS), specimens were tempered in the temperature range of 520–720 °C for 3 h followed by air cooling and an optimized tempering temperature was chosen to prolong holding time from 3 to 12 h. After heat treatments, microstructure examination was conducted by scanning electron microscope, X-ray diffraction examinations, hardness measurements and tensile tests. The results revealed that the superior mechanical properties were achieved by quenching at 1040 °C for 1 h+water cooling and tempering at 600 °C for 3 h + air cooling. Increasing isothermal tempering time could improve the toughness notably. It was believed that the property was correlated with the microstructure of tempered lath martensite and retained austenite. More retained austenite content is beneficial to the higher toughness of the SMSS.

Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel

Abstract: The effect of tempering on the microstructure and mechanical properties of a non-quenched (NQ) bainitic steel was investigated by optical microscopy, X-ray diffractometry, scanning electronic microscope (SEM) and transmission electronic microscopy (TEM). The results show that the NQ steel which has been investigated is granular bainite composed of bainitic ferrite lath, retained austenite film and island of austenite and martensite (M/A island) before tempering. The amount of retained austenite decreases with the rise of tempering, slowly before 400 °C, sharply at 450 °C, and it is close to 0 at 600 °C. When tempered at 350 °C, this kind of NQ steel has the optimum mechanical properties because of M/A islands partly decomposition, especially for the martensite in the M/A islands. However, when tempered at 450 °C, it appears brittleness, which results from carbides distributing along prior austenite grains because M/A island is largely decomposed.

Effect of microstructure and hardness on non-proportional cyclic hardening coefficient and predictions

Abstract: This paper investigates the effects of microstructure and hardness on non-proportional cyclic hardening of metallic materials. Constant amplitude in-phase and 90° out-of-phase strain-controlled axial-torsion cyclic tests were conducted to evaluate the hardening. Tubular specimens made from 1050 steel in normalized, quenched and tempered, and induction hardened conditions as well as 304L stainless steel were used to study the effect of microstructure on multiaxial cyclic deformation. Reductions in the non-proportional cyclic hardening were observed as the microstructure of 1050 steel changed form pearlitic–ferritic with lower hardness to tempered martensite with higher hardness. Significant non-proportional cyclic hardening was also observed for 304L stainless steel with austenitic microstructure. Multiaxial data generated in this study as well as multiaxial deformation data of several materials from literature suggest non-proportional cyclic hardening can be related to uniaxial cyclic hardening. Non-proportional hardening coefficients predicted from a proposed equation based on this observation were found to be in very good agreement with the experimental values in this study and from the literature.

Microstructure and failure behaviour of resistance spot welded DP980 dual phase steel

Abstract: In this research, microstructure and overload failure behaviour of resistance spot welded DP980 were investigated. Microstructural characterisation, microhardness test and static tensile shear test were conducted. Fusion zone size proved to be the most important controlling factor of spot weld peak load and energy absorption. The results of this study demonstrated that the conventional weld size recommendation of d=4t1/2 is not sufficient to ensure the pullout failure mode for DP980 steel resistance spot welds during the tensile shear test. In pullout mode, generally, failure was initiated at heat affected zone/base metal interface, where softening occurs due to the tempering of martensite. However, when heavy expulsion occurs, pullout failure tends to be initiated at fusion zone/heat affected zone interface. It was shown that heavy expulsion and associated large electrode indentation can reduce load carrying capacity and energy absorption capability of DP980 spot welds.

Composite magnetic recording medium

Abstract: A method for manufacturing glass hard disk substrates comprises annealing and then tempering previously formed glass hard disk substrates The annealed and tempered glass hard disk substrates have improved strength and stress resistance without chemical treatments

Hydrogen Embrittlement of Hardened Low-carbon Sheet Steel

Abstract: Tensile specimens of 10B22 (22MnB5) sheet steels were austenitized, quenched to martensite, and tempered at temperatures between 150 and 520°C for various times. The heat treated specimens were charged with 1.7 ppm hydrogen and immediately tested. Fracture surfaces were examined by field emission scanning electron microscopy. As-quenched martensitic specimens exhibited the most severe embrittlement and failed by stress-controlled cleavage fracture at low stresses. The initiation of hydrogen-induced fracture in specimens tempered between 150 and 350°C was consistent with glide plane decohesion, and coarse inclusion particles served as sources of hydrogen for circular areas of hydrogen-induced cleavage. Specimens tempered at 460 and 520°C showed little sensitivity to hydrogen embrittlement. The progression of decreased sensitivity to hydrogen-induced fracture with increasing tempering temperature correlates with the reduction in dislocations, the principal hydrogen traps, and the formation of cementite particles, considered to be ineffectual traps, with increased tempering. Very small amounts of intergranular fracture were observed, only in as-quenched specimens, confirming that boron has little effect on hydrogen embrittlement of hardened low-carbon steels.

The influence of temper shape on the mechanical properties of archaeological ceramics

Abstract: The influence of tempering on the mechanical performance of pottery is assessed. Emphasis is placed on the examination of the impact of temper shape (low vs. high sphericity) and extent of vitrification on the strength and toughness of the ceramic material. Measurements on experimental briquettes show that the replacement of platy phyllitic temper with bulky granitic temper results in a reduction in strength but, at least at very high firing temperatures, in an increase in toughness. The observed differences in strength reduction with different temper shapes is semi-quantitatively assessed by adapting the damaged zone model, developed previously to disk-shaped particles. The effect of temper shape on toughness is particularly pronounced at high firing temperatures, where platy temper results in significantly lower fracture toughness of the corresponding material.

Carbide characterization in a Nb-microalloyed advanced ultrahigh strength steel after quenching-partitioning-tempering process

Abstract: Based on the observations of scanning electron microscopy and transmission electron microscopy, four kinds of carbides were identified in a Nb-microalloyed steel after quenching–partitioning–tempering treatment. In addition to transitional epsilon carbide that usually forms in silicon-free carbon steel, other three types of niobium carbides (NbC) formed at various treatment stages respectively. They are incoherent NbC inclusion that nucleated at solidification mainly, fine NbC that nucleated in lath martensite at tempering stage and regular polygonal NbC that nucleated in austenite before quenching. Their formation mechanisms on steel were discussed briefly based on thermodynamics.

Premature creep failure in strength enhanced high Cr ferritic steels caused by static recovery of tempered martensite lath structures

Abstract: The apparent activation energy for rupture life sometimes changes from a high value of short-term creep region “H” to a low value of long-term creep region “L”. This change results in overestimation of rupture life during long-term creep. This is called “premature creep failure”. Premature creep failure has been studied by the evaluation of microstructures of three kinds of Gr. 122 type steels and by paying attention to subgrains recovery of tempered martensite lath structures. Subgrain structures remain unchanged during short-term aging, whereas static recovery of subgrain structure proceeds when diffusion distance during aging becomes sufficiently long. Strain-induced recovery is the dominant recovery mechanism in the short-term creep region, whereas the contribution of static recovery of the subgrain structures proceeds as well as strain-induced recovery in the long-term creep region. Hence, static recovery brings about premature creep failure. Static recovery of the subgrain structures can be evaluated well at all temperatures by hardness measurement of aged microstructures. Static recovery after incubation causes a drop in hardness due to the loss of subgrain boundaries as main obstacles against motion of dislocations. The less serious premature creep failure in 9% Cr steel as compared to that in 12% Cr is related to the slower speed of static recovery of the subgrain structure in 9% Cr steel. The static recovery is caused by the loss of particle pinning mainly due to MX and M 23 C 6 precipitates and controlled by their thermal stability. The spacing of MX does not change considerably after long-term aging for 9 and 10.5% Cr steels. Therefore, the static recovery of dislocation substructure cannot be attributed to the disappearance of MX particles, though the disappearance of MX can accelerate the recovery of the dislocation substructure in 12% Cr steel. The static recovery is controlled by the loss of strengthening due to M 23 C 6 precipitates. The M 23 C 6 spacing increases slowly in the 9% Cr steel, corresponding to the slow static recovery of its tempered martensite lath structure. The stability of M 23 C 6 precipitates may be affected by the change of chemical composition in M 23 C 6 particles during high temperature exposure.

Evolution and Coarsening of Carbides in 2. 25Cr-1Mo Steel Weld Metal During High Temperature Tempering

Abstract: Transformation and coarsening of carbides in 2.25Cr-1Mo steel weld metal during tempering at 700 °C for different time intervals ranging from 1 to 150 h were examined by transmission electron microscopy and scanning electron microscopy. M3C carbides were observed in the as-welded specimens and when tempered, the precipitates were mainly composed of M3C, M7C3, and M23C6 carbides. A sequence for corresponding carbide transformation during tempering with initial precipitation of M3C and the subsequent precipitation of M7C3 and M23C6 was proposed. The precipitation of M7C3 with higher chromium content was the main factor contributing to the decrease in coarsening rate of precipitates after prolonged tempering. The decrease in hardness of the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening owing to the coarsening of carbides.