P. Parameswaran

Prediction of microstructural states in Cr-Mo steels using phase evolution diagrams

The present paper introduces a new concept of phase evolution diagram and demonstrates its utility in predicting the subsequent microstructural states and assessing the previous thermal history, based only on the information of the composition of the parent matrix. The phase evolution diagram depicts the time variation of the solute composition of the parent proeutectoid ferrite matrix. The different metastable phase fields which evolve as a consequence of the variation in the solute composition are also superimposed in the diagram. The phase evolution diagram is constructed based on the extensive analytical electron microscopy studies on the precipitation of secondary carbides in Cr-Mo steels. The paper discusses the underlying principle, details of evaluation of the required experimental data and usefulness of the phase evolution diagram.

Finite element analysis of type IV cracking in 2.25Cr–1Mo steel weldment based on micro-mechanistic approach

Creep studies were carried out on 2.25Cr–1Mo steel base metal and its fusion-welded weldments at 823 K over the stress range 100–240 MPa. The weldment possessed lower creep rupture strength than the base metal due to type IV failure at the outer edge of the heat-affected zone (HAZ). Premature failure of the weldment was associated with pronounced creep cavitation accompanied with localized creep deformation in the soft intercritical region of the HAZ that was sandwiched between relatively higher creep deformation-resistant microstructural regions. The cavitation was associated with coarse intergranular precipitates in the intercritical region of the HAZ. The type IV cracking in the intercritical region of the HAZ was found to initiate deep inside the weldment and propagate towards the specimen surface. Finite element analysis of stress and strain distributions across the weldment was carried out considering the micro-mechanical strength inhomogeneity across it to explain the observed features of type IV cracking. The estimated higher von-Mises and principal stresses deep inside the intercritical region of the HAZ of the weldment led to the localized creep deformation and preferential cavity nucleation and growth, resulting in type IV failure of the weldment. The role of intergranular precipitate particles in the intercritical region of the HAZ in facilitating creep cavity nucleation by the exhaustion of creep ductility of the material close to the precipitate was corroborated from finite element analysis of stress and strain distribution around the precipitates.

Decomposition modes of austenite in CrMo ferritic steels

Abstract Experimental studies have been carried out on the decomposition modes of austenite in two varieties of ferritic steels. The variations in microstructural parameters with cooling rate and composition are explained in terms of thermal strain associated with cooling. A thermodynamic rationale is discussed to explain the difference in the driving force of austenite to martensite transformation in the steels under consideration.

Effect of nitrogen content on dynamic strain ageing behaviour of type 316LN austenitic stainless steel during tensile deformation

Abstract The effect of nitrogen content on the dynamic strain ageing (DSA) behaviour of type 316LN austenitic stainless steel has been studied. The nitrogen content was varied from 0·07 to 0·22 wt-%. The tensile tests were carried out over a temperature range of 300–1123 K and at three strain rates in the range 3×10−3–3×10−5 s−1. Serration was observed in the load elongation curves in the intermediate test temperature range and has been considered due to DSA phenomenon. The critical strain to onset of serrated flow increased with increase in nitrogen content and strain rate. The temperature for onset of DSA and the temperature of disappearance of DSA were found to increase with the increase in nitrogen content. The variations in tensile strength and work hardening rate of the steel with temperature exhibit peak values in the intermediate temperature range and have been attributed due to DSA phenomenon. The activation energy for DSA, estimated based on the temperature and strain rate dependences of the strain to onset of serrated flow, was found to increase from 111 to 218 kJ mol−1 with the increase in nitrogen content from 0·07 to 0·22 wt-% and the increase has been attributed to the possible enhanced interaction of the DSA causing interstitial nitrogen with substitutional chromium.

Influence of carbon content on microstructure and tempering behaviour of 2 1/4 Cr 1 Mo steel

Transmission electron microscopic studies aimed at elucidating the effect of carbon level on the tempering behaviour of 2 1/4 Cr 1 Mo steels have been carried out. Specimens with two different carbon levels (0.06% and 0.11 %) were cooled in flowing argon gas (AC) from an austenitization temperature of 1323 K and tempered at 823, 923 and 1023 K for times ranging from 2 to 50 h. The tempering behaviour at these temperatures for the two carbon levels is found to differ in the nature of secondary hardening at lower temperatures, variation in the time to peak hardness and the saturation level of hardness at long tempering times. Based on a detailed study, using analytical electron microscopy, on the morphology, crystallography and microchemistry of secondary phases, the factors governing the observed variations in tempering behaviour are related to the difference in the dissolution rate of bainite, nucleation of acicular M2C carbides and transformation rate of primary carbides into secondary alloy carbides. The carbides which promote softening were identified as M7C3, M23C6 and M6C, whereas hardening is mainly imparted by M2C.

Study of Microstructure and Property Changes in Irradiated SS316 Wrapper of Fast Breeder Test Reactor

This paper presents the results of a study on mechanical properties and microstructure in a SS316 wrapper irradiated in a 40MWt/13MWe fast breeder test reactor at Kalpakkam, India. Transmission electron microscopy (TEM) examination and mechanical property evaluation were carried out on the hexagonal wrapper subjected to different displacement damages up to a maximum of 83 dpa at an operating temperature of about 673 K. The steel irradiated to 83 dpa showed an increase in the yield strength and ultimate tensile strength, with a reduction in uniform elongation to about 8 %–10 % from about 20 % in the unirradiated material. Density measurements on the specimen from different portions of the irradiated wrapper showed a peak volumetric swelling of about 3.5 % at a damage of 83 dpa. TEM studies showed extensive void formation at 40 dpa and beyond in addition to extensive precipitation and formation of dislocation loops. The void density and size showed a progressive increase with displacement damage. The precipitates were identified to be of nickel and silicon enriched M6C type of η phase, while radiation induced G phase was also observed at 83 dpa. The increase in strength and reduction in ductility with increase in dpa is attributed to irradiation hardening, which is supported by the increase in defect density as observed from the dislocation substructure and increase in the void density and size. The paper would discuss the degradation in mechanical property in terms of the microstructural changes.

Small Specimen Test Techniques for Estimating the Tensile Property Degradation of Mod 9Cr-1Mo Steel on Thermal Aging

The degradation in mechanical properties of modified (mod) 9Cr-1Mo steel on thermal aging at 923 K has been studied using the small specimen test techniques—shear-punch and ball-indentation tests. Small volumes of material required for these test techniques make them unique tools for the assessment of service related degradation and failure analysis of structural components. A variety of heat treated microstructural conditions of mod 9Cr-1Mo steel is generated on which both conventional tensile test and shear-punch/ball-indentation tests are carried out. The relationship between conventional tensile properties and small specimen test results is established from these standardization experiments. Small specimens of Mod 9Cr-1Mo steel in the thermally aged conditions are tested using both shear punch and ball-indentation techniques and the tensile property changes are determined. Both these tests identically reflect and quantify the changes in the tensile properties with aging time which correlates well with the microstructural changes observed using optical and electron microscopic studies.

Effect of Capping Front Layer Materials on the Penetration Resistance of Q&T Steel Welded Joints Against 7.62-mm Armor-Piercing Projectile

In the present investigation, an attempt has been made to study the effect of capping front layers on the ballistic performance of shielded metal arc-welded armor steel joints which were fabricated with a chromium carbide-rich hardfaced middle layer on the buttered/beveled edge. Two different capping front layer materials were chosen for achieving better ballistic performance, namely, low hydrogen ferritic (LHF) and austenitic stainless steel (SS) fillers. On the other hand, the bottom layers are welded with SS filler for both joints. The consequent sandwiched joint served the dual purpose of weld integrity and penetration resistance of the bullet. It is observed that the penetration resistance is due to the high hardness of the hardfacing layer on the one hand and the energy-absorbing capacity of the soft backing SS weld deposits on the other hand. The complementary effect of layers successfully provided resistance to the projectile penetration. On a comparative analysis, the joint fabricated using the LHF filler capping front layer offered superior ballistic performance with respect to depth of penetration. This is mainly due to the presence of acicular ferrite along the bainitic structure in the LHF capping front layer, which caused a shallow hardness gradient along the weld center line.

The Influence of Prior Microstructure on Tempering Stages in 2.25Cr-1Mo Steel

The influence of prior microstructure on the tempering behaviour of 2.25Cr-1Mo steel at two different temperatures is reported in this paper. The tempering behaviour of the steel could be classified into four different stages. The prior microstructure, namely the amount of bainite, was found to influence the following three characteristics: 1) the extent of secondary hardening, 2) time to reach peak hardness and 3) the saturation level of hardness. The observed influence of prior microstructure on tempering behaviour has been understood in terms of the synergistic effect of a number of microstructural changes, like the modification of bainite, growth of a number of carbides and the recovery processes. The paper discusses the influence of two different prior microstructures on the above features of tempering behaviour, at two temperatures.

Effect of Normalization Heat Treatment on Creep and Tensile Properties of Modified 9Cr–1Mo Steel

AbstractCreep and tensile properties have been investigated on modified 9Cr–1Mo steel subjected to single and double normalization heat treatments. Optical, scanning and transmission electron microscopic investigations revealed the presence of finer prior austenite grain size and coarsening of MX precipitates in the double normalized steel as compared to the steel subjected to single normalization heat treatment. Increased minimum creep rate and significant reduction in creep rupture life were observed in the double normalized steel. Increased tensile ductility coupled with marginal decrease in tensile strength was observed with double normalized steel compared to single normalized steel. The double normalized steel exhibited lower creep strength and higher tensile ductility than the single normalized steel.