K.K. Ray

Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel

The influence of deep cryogenic processing in between quenching and tempering (QCT) on the carbide precipitation and the tribological behavior of a commercial AISI D2 steel has been examined. The developed microstructures have been characterized with an emphasis to understand the influence of QCT vis-à-vis conventional quenching and tempering (QT) on the nature, size, morphology, and distribution of carbide particles. The mechanical properties such as hardness and wear resistance of the samples treated by QT and QCT have been evaluated employing Vickers indentation and sliding wear techniques, respectively. It has been demonstrated that deep cryogenic treatment leads to considerable microstructural changes which result in enhanced tribological properties.

On the enhancement of wear resistance of tool steels by cryogenic treatment

The present article aims to resolve the debate on the degree of benefit of processing tool or die steels by cryogenic treatment. This has been done by measuring transition loads for quenched and tempered die steel with and without cryogenic treatment. The wide range of reported degrees of improvement of wear resistance by cryotreatment is explained by the operative modes and mechanisms of sliding wear considering the wear rate ratio of conventionally treated to cryotreated steels on a two-dimensional map.

Correlation between ultrasonic shear wave velocity and Poisson’s ratio for isotropic solid materials

Abstract A new correlation between ultrasonic shear wave velocity and Poisson’s ratio has been established for isotropic solid materials, based on the data generated experimentally and collected from the literature. Poisson’s ratio has been found to decrease with increasing ultrasonic velocity in various solid materials such as metals and alloys, ceramics and glasses, intermetallics and polymers. The slope of the plot of the ultrasonic velocity against Poisson’s ratio is found to be almost constant for any given alloy system with different microstructures associated with various heat treatments, alloying elements, grain size, temperature effect, etc. Further, it has been demonstrated that ultrasonic shear wave velocity is a better parameter for materials characterization as compared to longitudinal wave velocity.

On the refinement of carbide precipitates by cryotreatment in AISI D2 steel

Refinement of carbide particles by cryotreatment is often proposed as a major factor for the improvement of wear resistance in tool steels. However, this proposition is not substantiated by experimental evidence. This has been examined in this report by (i) detailed micro-structural analyses of the nature, volume fraction, size, population density and distribution of carbide particles, (ii) XRD and EDX micro-analysis on the bulk samples and electrochemically extracted carbides, and (iii) measurement of hardness and wear rate of a series of differently cryotreated AISI D2 steel. The results conclusively establish that (i) cryotreatment, in comparison to conventional treatment, induces precipitation of finer carbides with higher volume fraction and more uniform distribution, and (ii) population density and the size of secondary carbide particles significantly increases with holding time up to a critical duration at 77 K in cryotreatment. The latter observation indicates the pioneering direction towards optimization of cryotreatment design for techno-economic benefit.

Effect of prestrain on fracture toughness of HSLA steels

The effect of tensile prestraining on the ductile fracture behaviour of two varieties of Cu-strengthened HSLA steels has been studied. It was observed that for both the HSLA steels the fracture toughness, JC, remained invarient up to 2% prestrain, beyond which it decreased deleteriously. The nature of variation of fracture toughness with prestrain has been explained in terms of variation of the tensile flow properties of the steels with prestrain. A model correlating fracture toughness and the tensile flow properties has been proposed and it is demonstrated that the predictions of the model matched closely with the experimental observations.

Characterization of solutionizing behavior in VT14 titanium alloy using ultrasonic velocity and attenuation measurements

VT14 titanium alloy (Ti-4.5Al-3Mo-1V) was subjected to a series of heat treatments consisting of solutionizing for 1 h at the selected temperatures in range of 923-1323 K at an interval of 50 K, followed by water quenching. Hardness and optical microscopy results are correlated with ultrasonic longitudinal and shear wave velocities and attenuation in these specimens. Ultrasonic velocities and hardness decrease with solution annealing temperature (SAT) in the 923-1123 K range. Beyond 1123 up to 1223 K, they increase slightly. Beyond 1223 K, ultrasonic velocities become constant, whereas hardness increases up to 1323 K. Ultrasonic attenuation exhibits an opposite behavior to velocity and hardness. Further, for the first time, authors have shown that ultrasonic velocity can be used to identify the β-transus temperature in this alloy. Because of non-monotonous variation of velocity and attenuation with solutionizing temperature, it was not possible to identify the SAT using any one of these parameters. Hence, a new parameter, ratio of normalized differential of ultrasonic attenuation to normalized differential of ultrasonic velocity (RNDAV) has been used, which is found to increase monotonously with SAT and hence enabling unambiguous characterization of SAT in solution annealed VT14 alloy.

Study of cast microalloyed steels

Abstract Improved grades of cast steels were prepared by microalloying C–Mn–Cr steels (0.15–0.22C, wt-%) with V (0.1)+Ti (0.01) or V (0.11)+Nb (0.034)+Ti (0.01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784–1078 MPa (80–110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0.9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the laths into polygonal ferrite occurred. Fine precipitates were deposited on the dislocation substructure.

Inconsistent wear behaviour of cryotreated tool steels: role of mode and mechanism

Abstract This report aims to reveal the cause of wide variation in the reported degree of improvement in wear resistance of cryotreated tool steels. Sliding wear tests at different normal loads have been carried out on conventional and cryotreated AISI D2 steel specimens together with SEM examinations and EDX microanalyses of the surfaces and subsurfaces of the worn specimens and that of the generated debris. The obtained results reveal that when the modes and mechanisms of wear are similar for both types of specimens, mild oxidative at lower load or severe delaminative at higher load, the improvement in wear resistance is 1·6–2·2 times. At the intermediate load, the modes and mechanisms are dissimilar, and the observed improvement is as high as 53·2 times. The reported varied degree of improvement in wear resistance by cryotreatment has been attributed to the operating test conditions that govern the modes and mechanisms of wear.

Acoustic emission from tensile deformation of unnotched and notched specimens of AISI type 304 stainless steels

Abstract A set of experiments has been carried out to examine the characteristics of the acoustic emission (AE) generated during tensile deformation of AISI type 304 stainless steels. Two grades of steels—nuclear and commercial; and two types of specimens—unnotched and notched, have been used in this study. The AE total counts obtained in different specimens for specific strain levels have been analysed by regression analysis approach of data analysis. The results indicate that the AE counts from notched specimens are usually higher than those from unnotched specimens in the nuclear grade steel, unlike the opposite results obtained for the commercial grade steel in the present study and also reported results for other materials. These results have been explained with the help of the different sources of AE in the two grades of stainless steels.

The load dependence of hardness in alumina–silver composites

The room temperature Knoop micro-hardness properties of three alumina-silver composites vis-a-vis a monolithic alumina have been investigated at different indentation loads. The composites have been developed using conventional pressureless sintering at 1550°C and these consisted of 1.6, 3.38 and 5.01% by volume of silver particles. It has been shown that the measured Knoop hardness values of these materials sensitively depend on the applied load of indentation. But following an emerging analytical technique, the composites can be assigned some unique true hardness values, which are functions of their silver content.