Secondary Hardening Temperature Research Articles

Developing high strength/high toughness grades steels by dual-precipitates co-configuration during aging process

Development of new ultrahigh strength steels with high strength and high toughness requires thorough comprehension of nanoscale precipitation mechanisms. In this investigation, a comprehensive array of techniques including atom probe tomography, transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction, small angle x-ray Scattering and thermodynamic calculations were employed. These methods unveiled an intriguing co-precipitation mechanism involving NiAl and carbide nanoparticles in a 2.2 GPa grade strength steel. The results demonstrate that the precipitation mechanisms of NiAl and carbides at different aging temperatures have a significant impact on the strength and toughness of the steel. Sub-micron ε-carbide at the interface of martensite lath and uniformly distributed NiAl cluster in the matrix are independent nucleation during the low-temperature aging process. The high distribution density of NiAl cluster in the matrix results in an increase in the impact absorbed energy of the steel to 136 J, albeit with a slight decrease in strength. On the other hand, the NiAl with minimal lattice misfit and low interfacial energy preferentially nucleates uniformly in the matrix, inducing the nucleation of M2C and increasing the number density of M2C precipitates at the secondary hardening temperature. The homogeneous precipitation of spherical NiAl, along with needle-shaped M2C in the martensite, significantly enhances the strength of the steel, with a tensile strength reaching up to 2216 MPa and an impact absorbed energy of 21 J.

Comprehensive analysis of nitriding behavior and synergistic strengthening mechanism on quenched M50 steel

M50 secondary hardening steel is widely used as high-end bearing steel for engine shafts in the aerospace industry. In this study, plasma nitriding at different temperatures varying from 410 to 570 °C was performed on quenched M50 steel to investigate the response of the microstructure and mechanical properties to nitriding temperature and advance the scientific understanding of the mechanism involved. The nitrided samples were fully characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron microprobe analysis, Vickers hardness tester, and Pin-on-disc tribometer. The results revealed that the nitrided layer's thickness, microstructure, hardness, and wear resistance depended on the treatment temperature. When the nitriding treatment performs at secondary-hardening temperature, the samples were endowed with excellent wear resistance and gentle hardness profile, i.e., hardness profile with a gradually reducing hardness profile towards the core, attributed to the synergistic strengthening. Moreover, the thermodynamic calculation was involved to clarify the formation mechanism of coarse intergranular precipitation, which can be connected to form a wave shape line at higher temperatures.

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

A PM made Cr-V ledeburitic tool steel Vanadis 6 has been subjected to conventional austenitizing and quenching, which was followed by sub-zero treatment at different temperatures, and by tempering treatments. The microstructure, hardness and fracture toughness of sub-zero treated steel have been investigated with reference to the same material after conventional room temperature quenching. The main findings are that sub-zero treatments reduce the retained austenite amount, enhance the population density of small carbides, refine the martensite and change the precipitation of carbides during tempering. These alterations are reflected in elevated hardness after low-temperature tempering but slightly lowered hardness after tempering within the normal secondary hardening temperature range, except the specimens treated at-140 °C where the hardness improvement was maintained. The fracture toughness is rather negatively influence by the sub-zero treatments, except the treatment at-140 °C where no impact or rather improvement has been recorded; thus, the treatment at a temperature of-140 °C seems to be a promising way how to improve the hardness and the fracture toughness pf the Vanadis 6 steel simultaneously.

Microstructure – Toughness relationships in sub-zero treated and tempered Vanadis 6 steel compared to conventional treatment

The influence of sub-zero treatment and tempering on the microstructure, flexural strength, and fracture toughness of Cr-V ledeburitic steel was examined with reference to the same material processed without the sub-zero period. The experimental results lead to the following findings: (i) Sub-zero treatment induces a considerable reduction of the amount of retained austenite, refinement of newly formed martensite, and increase in population density of small globular carbides. (ii) The hardness is improved by the sub-zero treatment in the prior-to-tempering state as well as in the state after low-temperature tempering; after high-temperature tempering, the situation is changed in favour of conventionally quenched specimens, whereas the secondary hardening peak disappears in sub-zero treated steel. This is due to combined effect of the population density decrease of small globular carbides, accelerated decomposition rate of retained austenite, coarsening of cementitic precipitates, and retardation of precipitation of stable M7C3 carbide. (iii) There is almost no effect of sub-zero treatment on the flexural strength in the low-temperature tempering range but this property is improved after tempering into the secondary hardening range. Sub-zero treatment causes a worsening of fracture toughness after low-temperature tempering but a slight improvement after tempering in the secondary hardening temperature range.

The effect of subzero treatment on microstructure, fracture toughness, and wear resistance of Vanadis 6 tool steel

Vanadis 6 ledeburitic steel manufactured by powder metallurgy of rapidly solidified particles was conventionally quenched and subzero-treated at temperatures of −90 and −196°C for 4 and 10h, respectively, followed by standard double-tempering to a secondary hardening peak at 530°C for 2h. Besides quantification of standard microstructural features by using examination by scanning electron microscopy and X-ray diffraction analysis, the fracture toughness, hardness and nanohardness, flexural strength, fracture toughness, and wear resistance have been characterized. The obtained results indicate the following: (i) The microstructure of the material consists of tempered martensite and eutectic, secondary, and small globular carbides. Retained austenite is completely eliminated by application of the given heat-treatment schedules. (ii) The amount of small globular carbides is significantly increased by subzero treatments; the lower the temperature (or the longer the duration) of subzero treatment, the higher the content of these carbides. (iii) The hardness of the material is influenced slightly negatively by subzero treatments followed by tempering in the normal secondary hardening temperature range. (iv) There is no negative impact of the given treatment schedules on either toughness or fracture toughness of the steel. (v) When a hardened ball bearing is used as a counterpart, wear resistance is improved by subzero treatments despite the slightly lower hardness of the examined tool steel.

Characterization of Electroslag Remelted Super Hard High Speed Tool Steel Containing Niobium

AbstractIn this work, thermodynamical equilibria have been calculated for super hard high speed tool steel (HSS), AISI M41 composition belonging to the multicomponent system Fe–C–Cr–W–Mo–V–Co as well as for its three different variant containing niobium. Some temperature‐concentration diagram for both class of alloys are presented and calculated quantities (melting and transformations, amount and composition of phases) are compared with experimental data. Effect of niobium and high cooling rate during electroslag remelting on the precipitated carbides were discussed. In this work, the effect of niobium on morphology of carbides secondary hardening temperature and wear rate of investigated HSSs were studied. This work aims to study the effect of niobium as alloying element on precipitated carbides type and shape, in addition to study the effect of precipitated carbides after full heat treatment on secondary hardening and wear resistance for investigated steels.

Effect of Nitrogen and Niobium on the Structure and Secondary Hardening of Super Hard High Speed Tool Steel

AbstractHigh‐speed steels have been used mostly for multi‐point cutting tools and for plastic working tools. High speed steels are ferrous based alloys of the Fe‐C‐X multi‐component system where X represents a group of alloying elements comprising mainly Cr, W or Mo, V, and Co. The properties of these steels can be improved by modifying their chemical composition or the technology of their production. One of the new trends in modifying the tool steels chemical composition consists in the addition of niobium and nitrogen. In this work, the effects of niobium and nitrogen on morphology of carbides and secondary hardening temperature of investigated high speed tool steels were studied. This experimental work shows that, the conventional ingots have many types of carbides of different shapes and sizes precipitate on the boundary together with thick needle like carbides. On the contrary, for nitrogen steel, the nitrogen alloying leads to form dense, fine and well distributed microstructure. While, on the case of niobium alloying, single carbide (MC), and different types of eutectic carbides were precipitated which have a major effect on the secondary hardening temperature.

Tempering behavior of a semi-high speed steel containing nitrogen

The decomposition behavior of martensite and precipitation characteristics of carbonitrides during the tempering process of a low-alloy high-speed steel (semi-HSS) containing nitrogen were investigated by means of transmission electron microscopy, scanning electron microscopy, energy dispersive spectrometry and electron energy loss spectroscopy. The results shown the secondary hardening effect is reinforced with addition of nitrogen and the secondary hardening temperature range has expanded. The stability of retained austenite is also studied. When tempered in the temperature range of 425–525 °C, large quantities of fine-scale V-rich nitride precipitations distribute in the matrix homogeneously. A complex carbonitride phase can form when tempered at 550 °C.

Fatigue Strength of Laser-processed Hot Work Tool Steel.

Hot work tool steel with an application in the die-casing industry was chosen as the test material. Most of the failure of dies depends on the heat check due to cyclic thermal stress. So, the present work aims to find a repair method of the failed tool. The microstructure of the Nd:YAG laser-processed material is formed by dendritic structure hardened into martensite. The steel shows secondary hardening by tempering and fatigue softening by cyclic loading even at room temperature. The rotating bending fatigue strength can be recovered to almost initial value by heat treatment at secondary hardening temperature after laser-processing. As the result, it is concluded that the laser processing is suitable for the repair of heat check.

Hydrogen Trapping Behavior in Vanadium-added Steel

Hydrogen trapping and de-trapping behavior was investigated for steels with and without V. The de-trapping of hydrogen is very slow while the trapping presumably proceeds rapidly for steels containing VC precipitates. The activation energy for de-trapping is in the range of 33 to 35 kJ/mol. The trapped-hydrogen content and diffusible-hydrogen content in the steady state increase with increasing hydrogen entry rate into the steel. The density of hydrogen trapping sites decides the maximum trapped-hydrogen content; 9 ppm for 1 % V steel tempered at peak secondary hardening temperature. Analysis of hydrogen embrittlement cracking tests in terms of hydrogen contents such as the critical hydrogen content should be performed on the specimens with uniform hydrogen distribution and must consider the nature of hydrogen whether it is trapped or diffusible.

The effect of tungsten on dislocation recovery and precipitation behavior of low-activation martensitic 9Cr steels

The effect of W on dislocation recovery and precipitation behavior was investigated for martensitic 9Cr-(0,l,2,4)W-0.1C (wt pct) steels after quenching, tempering, and subsequent prolonged aging. The steels were low induced-radioactivation martensitic steels for fusion reactor structures, intended as a possible replacement for conventional (7 to 12)Cr-Mo steels. During tempering after quenching, homogeneous precipitation of fine W2C occurred in martensite, causing secondary hardening between 673 and 823 K. The softening above the secondary hardening temperature shifted to higher temperatures with increasing W concentration, which was correlated with the decrease in self-diffusion rates with increasing W concentration. Carbides M23C6 and M7C3 were precipitated in the 9Cr steel without W after high-temperature tempering at 1023 K. With increasing W concentration, M7C3 was replaced by M23C6, and M6C formed in addition to M23C6. During subsequent aging at temperatures between 823 and 973 K after tempering, the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains occurred. Intermetallic Fe2W Laves also precipitated in the δ-ferrite grains of the 9Cr-4W steel. The effect of W on dislocation recovery and precipitation behavior is discussed in detail.

Implication of elastic coherency in secondary hardening of high Co-Ni martensitic steels

Composition and stoichiometry of coherent M2C carbide precipitates, (M=Cr, Mo, Fe), in two high Co-Ni martensitic steels have been calculated using thermodynamic conditions for mechanical, chemical and interfacial equilibrium between the ferrite (tempered martensite) and the M2C phase. Compared to the corresponding incoherent equilibrium compositions, our calculations at the standard secondary hardening temperature of 783 K predict a substantial carbon deficiency and a measurable solubility of iron in the M2C phase, both of which have been verified experimentally. These deviations from the incoherent equilibrium compositions are found to be in line with the influence of these constituents in lowering the strain energy by reducing the principal strains of the M2C stress-free transformation strain.

Carbides in long-tempered vanadium steels

Study with the electron microscope of the carbides in vanadium-chromium-molybdenum steels after tempering up to 1000 hr at 600 to 1400°F confirmed that alloy carbides are formed at the secondary hardening temperature by decomposition of the plate-like iron carbides. It was also demonstrated that vanadium carbide persists as much smaller particles than do chromium- or molybdenum-bearing carbides. Conditions conducive to the formation of fine vanadium carbides are indicated to be favorable for high temperature strength.