Relationships between the Microstructure, Hardness and Fracture Toughness of Differently Sub-Zero Treated Tool Steel

Achieving high fracture toughness and tribological properties in high-carbon steels via sub-zero treatment and low-temperature tempering

The microstructure, phase constitution, and hardness of Cr‐V ledeburitic tool steel Vanadis 6 subjected to subzero treatment at −140°C and for different soaking times have been investigated. The light microscopy, scanning electron microscopy, and X‐ray diffraction have been used for microstructural investigations. The hardness has been evaluated by the Vickers method. The obtained results assist to draw that subzero treatment reduces the retained austenite amount and increases the population density of carbides, compared to conventional heat treatment. The extent of decrease in the retained austenite amount makes around 85%, and the increase in population density of small globular carbides was approximately fivefold. High compressive stresses were identified in the retained austenite, and their values follow the increase in carbide count. This makes a serious support to the theory explaining the formation of “extra” carbides as a by‐product of more complete martensitic transformation. As a result of the mentioned microstructural changes, the material hardness increased from 875 ± 16 HV 10 up to 954.6 ± 14 HV 10 for conventionally quenched and SZT steels, respectively.

The effect of sub-zero treatments on dry sliding wear performance of Vanadis 6 tool steel was investigated by pin-ondisc tests against three counterpart types: alumina, 100Cr6 ball bearing steel, and CuSn6 bronze. The microstructure of the examined steel consists of tempered martensitic matrix with very small amounts of retained austenite, and three carbide types. Sub-zero treatments increase the carbides count. The hardness of the steel increases with increasing the austenitizing temperature but slightly decreases with the application of sub-zero treatments. Sub-zero treatments affect the wear performance of the Vanadis 6 steel when tempered at the secondary hardening peak as follows: almost no effect of this kind of treatment was detected when alumina was used as a counterpart but moderate amelioration was recorded when either 100Cr6 steel or CuSn6 bronze was used in the sliding couple. This behaviour is attributed to the combined effect of hardness variations and changes in population density of carbides, due to application of sub-zero treatment.

Structure–property correlation of sub-zero treated AISI D2 steel

AISI D2 tool steel has become important material in the industry, especially for mold and dies maker and also used for cutting tools. Conventional hardening process such as annealing and quenching is normally done for this kind of steel as it needs high hardness and wear resistance properties. To further improve its performance, sub-zero treatment can be performed. Quenching the steel in very low temperature e.g. liquid nitrogen, will help to remove unwanted retained austenite, thus provide better dimensional stability. In this study, AISI D2 tool steel samples were subjected to austenitizing temperature of 1030°C and then quenched using different quenching media i.e. air, oil, and liquid nitrogen. Vickers test result showed that after quenching process, the hardness for those samples were 762 HV, 822 HV, and 735 HV for air, oil, and liquid nitrogen quenched respectively. Microstructure analysis showed that retained austenite volume fraction after quenching process were 7.50%, 3.93%, and 3.18% for air, oil, and liquid nitrogen quenched respectively.

Changes in microstructure of ledeburitic tool steel due to vacuum austenitizing and quenching, sub-zero treatments at −140°C and tempering

To evaluate critically Carl Wunderlich's axioms on clinical thermometry. Descriptive analysis of baseline oral temperature data from volunteers participating in Shigella vaccine trials conducted at the University of Maryland Center for Vaccine Development, Baltimore. Inpatient clinical research unit. One hundred forty-eight healthy men and women aged 18 through 40 years. Oral temperatures were measured one to four times daily for 3 consecutive days using an electronic digital thermometer. Our findings conflicted with Wunderlich's in that 36.8 degrees C (98.2 degrees F) rather than 37.0 degrees C (98.6 degrees F) was the mean oral temperature of our subjects; 37.7 degrees C (99.9 degrees F) rather than 38.0 degrees C (100.4 degrees F) was the upper limit of the normal temperature range; maximum temperatures, like mean temperatures, varied with time of day; and men and women exhibited comparable thermal variability. Our data corroborated Wunderlich's in that mean temperature varied diurnally, with a 6 AM nadir, a 4 to 6 PM zenith, and a mean amplitude of variability of 0.5 degrees C (0.9 degrees F); women had slightly higher normal temperatures than men; and there was a trend toward higher temperatures among black than among white subjects. Thirty-seven degrees centigrade (98.6 degrees F) should be abandoned as a concept relevant to clinical thermometry; 37.2 degrees C (98.9 degrees F) in the early morning and 37.7 degrees C (99.9 degrees F) overall should be regarded as the upper limit of the normal oral temperature range in healthy adults aged 40 years or younger, and several of Wunderlich's other cherished dictums should be revised.

Present work examines tool steels abrasion wear resistance and the abrasion mechanisms which are one main contributor to failure of tooling in metal forming industry. Tooling used in cutting and metal forming processes without lubrication fails due to this type of wear. In the workshop and engineering practice, it is common to relate wear resistance as function of material hardness only. However, there are others parameters which influences wear such as: fracture toughness, type of crystalline structure and the occurrence of hard precipitate in the metallic matrix and also its nature. In the present investigation, the wear mechanisms acting in tool steels were analyzed and, by normalized tests, wear resistance performance of nine different types of tool steels were evaluated by pin‐on‐disc testing. Conventional tool steels commonly used in tooling such as AISI H13 and AISI A2 were compared in relation to tool steels fabricated by sintering process such as Crucible CPM 3V, CPM 9V and M4 steels. Friction and wear testing were carried out in a pin‐on‐disc automated equipment which pin was tool steel and the counter‐face was a abrasive disc of silicon carbide. Normal load of 5 N, sliding velocity of 0.45 m/s, total sliding distance of 3000 m and room temperature were employed. The wear rate was calculated by the Archard’s equation and from the plotted graphs of pin cumulated volume loss versus sliding distance. Specimens were appropriately heat treated by quenching and three tempering cycles. Percentage of alloying elements, metallographic analyses of microstructure and Vickers microhardness of specimens were performed, analyzed and correlated with wear rate. The work is concluded by the presentation of a rank of tool steel wear rate, comparing the different tool steel abrasion wear resistance: the best tool steel wear resistance evaluated was the Crucible CPM 9V steel.

Increasing applications of AISI 4340 steel necessitate more precise characterization of its mechanical properties so that designers can better ensure failsafe behaviour in service. The fatigue fracture process of a component includes crack initiation and crack propagation. C r a c k initiation behaviour can be evaluated by studying low cycle fatigue and high cycle fatigue. The crack propagation life depends on fatigue crack growth rate and fracture toughness. Many researchers have conducted fracture toughness measurement [1-3], dynamic fracture propagation [4-6] and bending and torsion fatigue tests [7] in 4340 steel. It has been shown that the fracture toughness can be improved by using higher austenitizing temperature (1100-1200 °C) or through high temperature thermomechanical treatment [3]. However, little work has been done on the fatigue crack growth rate of 4340 steel [8]. The second phase plays an important role in determining the mechanical properties. The second phase hard carbide particles in a tool steel will develop stresses at the boundary of the matrix and the inclusion during external cycling loading [9] and reduce the low cycle fatigue properties. Zaczyk et al. [10] indicate that the absorbed energy during stable crack growth depends upon the amount the morphology of second phase particles in ductile steels. The resistance to shear instability and fracture toughness also relies on second phase particle dispersions [11]. Tomita and Okabayashi have proposed two heat treatments that can produce a dutile second phase in the martensitic matrix of 4340 steel [12-15]. The first process is isothermal heat treatment (IHT), which produces a lower bainite second phase, and the second is interrupt quench treatment (IQT), which produces a highly tempered martensite second phase. The existence of the ductile second phase can significantly improve the mechanical behaviour such as tensile properties, impact energy [12-14] and plane strain fracture toughness [15]. However, Tomita and Okabayashi did not address the fatigue properties. In the study reported here we investigated the effect of types and volume fraction of the second phase on the fatigue crack growth rate in AISI 4340 steel. The results are compared to those of conventional quenched and tempered 4340 steel. A commercial AISI 4340 steel was used in this study. The chemical composition was (wt%: C 0.39, Si 0.24, Mn 0.61, P 0.02, S 0.01, Ni 1.46, Cr 0.67, Mo 0.17). The martensitic transformation temperature Ms determined by a dilatometer was 330 °C. Based on the Ms temperature, three different heat treatments were carried out to produce ductile second phase in the matrix: isothermal heat treatment, interrupt quench treatment and direct quench treatment (DQT). Table I lists these heat treatment processes and specimen identification. The microstructures of specimens etched with a 3% nital solution were observed by optical microscopy. Fatigue crack growth tests in accordance with the ASTM E647 standard [16] were performed in a computerized MTS servo-controlled testing system. Compact tension specimens of thickness 5.4 mm and width 43.2 mm were used. A sinusoidal load with frequency 40 Hz and load ratio R = 0.1 was applied throughout the test. A crack opening displacement gauge was attached close to the notch of the specimen, and the growi~ag crack length was measured by the compliance method. Fatigue cycles and cyclic stress intensity factor at every moment of stressing could be recorded automatically. Two duplicate fatigue tests were performed for each specimen. The broken surfaces were studied by scanning electron microscopy (SEM) to understand

Rapid tempering of a medium-carbon martensitic steel: In-depth exploration of the microstructure – mechanical property evolution

In recent years, deep cryogenic treatment (DCT) is getting increased attention in many tooling applications as a mean to improve tool performance. However, contradictory results are reported indicating improved as well as deteriorated wear resistance. The goal of this work was to investigate effect of DCT on wear properties of tool steel and how they change depending on the tool steel type, and hardness and fracture toughness obtained. Results show that steel composition considerably affects the way how cryogenic treatment changes steel properties. For low carbon tool steel toughness properties can be improved for up to 70% at only minor hardness drop, but are quite limited in the case of high-speed steel, while at high carbon content both hardness and toughness are deteriorated. In general, DCT was found to result in reduced abrasive wear resistance except for cases of combined hardness and fracture toughness improvement.

Optimization of tool steel properties and corresponding heat treatment is mainly based on trial and error approach, which requires tremendous experimental work and resources. Therefore, there is a huge need for tools allowing prediction of mechanical properties of tool steels as a function of composition and heat treatment process variables. The aim of the present work was to explore the potential and possibilities of artificial neural network-based modeling to select and optimize vacuum heat treatment conditions depending on the hot work tool steel composition and required properties. In the current case training of the feedforward neural network with error backpropagation training scheme and four layers of neurons (8-20-20-2) scheme was based on the experimentally obtained tempering diagrams for ten different hot work tool steel compositions and at least two austenitizing temperatures. Results show that this type of modeling can be successfully used for detailed and multifunctional analysis of different influential parameters as well as to optimize heat treatment process of hot work tool steels depending on the composition. In terms of composition, V was found as the most beneficial alloying element increasing hardness and fracture toughness of hot work tool steel; Si, Mn, and Cr increase hardness but lead to reduced fracture toughness, while Mo has the opposite effect. Optimum concentration providing high KIc/HRC ratios would include 0.75 pct Si, 0.4 pct Mn, 5.1 pct Cr, 1.5 pct Mo, and 0.5 pct V, with the optimum heat treatment performed at lower austenitizing and intermediate tempering temperatures.

Annealing is a commonly used fabrication technology of graphene-assembled materials, which serves as an efficient method to control material properties. In graphene-assembled materials, the multilayer folded configuration of graphene has been widely observed due to the two dimensional characteristic of graphene. However, the manipulation on the mechanical properties of graphene-assembled materials by annealing has not been fully understood yet, especially considering the effect of folded microstructures. In this paper, we focus on the effect of annealing temperature on the mechanical properties of multilayer folded graphene. The dependences of elastic modulus, tensile strength, ultimate strain and fracture toughness on the annealing temperature have been systematically studied by molecular dynamics simulations. Moreover, the mechanisms behind the manipulations by annealing temperature have been revealed combining the structural evolutions obtained from the simulations. Our results indicate that the multilayer folded graphene after annealing under higher temperature exhibits significant reinforcement on its elastic modulus and tensile strength, while its ultimate strain drops instead. The fracture toughness is enhanced only within a certain range of annealing temperature. The controllable mechanical properties are attributed to the formation of interlayer covalent bonds between carbon atoms belonging to adjacent layers during the annealing processing. With the annealing temperature increases, more interlayer crosslinks are observed from simulations, which greatly strengthens the interlayer interaction. For the cases with lower annealing temperature, the folded graphene can be unfolded easily then finally flattened under tensile stretch, and the structural failure originates from the interlayer slippage in the folded area. However, for the cases with higher annealing temperature, the unfolding deformation is prevented since the folded graphene is blocked by much denser interlayer crosslinks, and the origins of structural failure transforms to the intralayer fracture in graphene plane. Considering the intralayer covalent bond interaction is far more powerful than the interlayer van der Waals interaction, the higher annealing temperature will bring higher elastic modulus and tensile strength due to the change on the structural failure mode, but it will sacrifice the ductility at the same time due to the blocked unfolding process of folded area. It is confirmed in our study that the annealing is an effective approach for the synthetic modulation on the stiffness, strength, ductility and toughness of multilayer folded graphene.

In Newell & Northcroft (1967) evidence was presented which showed that the metabolism of certain intertidal invertebrates at rest does not vary greatly with temperature change. The following work was undertaken to determine whether temperature‐independent metabolism could be demonstrated in suspensions of mitochondria extracted from poikilotherm tissue and whether it could be inferred that there was any difference in the ability of poikilotherms from different habitats to adjust toshort‐term temperature change.It was found that the rate of reaction/temperature curve of mitochondria isolated from the skate Raia had a flattened region below 10°C but above this the rate of oxidation of succinate and pyruvate increased regularly with temperature. The curve for intertidal forms had a gradual slope extending as high as 20°C in the mussel Mytilus edulis while in the terrestrial snail Helix aspersa the gradual slope reached 27.5°C after which the rate of oxidation of substrate increased sharply before declining towards higher temperatures. Finally, the curve for mitochondria isolated from the desert locust Schistocerca gregaria showed a gradual slope up to 35°C after which a sharp increase in the rate of oxidation of substrate occurred. Thus the extent of the gradual slope corresponds with the normal environmental temperature range to which the tissues of the poikilotherms are subjected and the general form of the R‐T curve is such that rapid fluctuations within the normal environmental temperature range have relatively little effect on the rate of mitochondrial activity.

Tribological properties of AlN [sbnd]CeO 2[sbnd]Si 3N 4 cutting materials in unlubricated sliding against tool steel and cast iron