Low Tempering Research Articles

Effects of alloy additions and tempering temperature on the sag resistance of Si–Cr spring steels

The effects of Mo, W, and V additions and tempering temperatures on the sag resistance in relation to the microstructural evolution during tempering of Si–Cr spring steels were investigated by performing TEM examination, torsional Bauschinger tests, hardness tests and tensile tests. The hysteresis loop area measured in torsional Bauschinger tests, which is closely related with the sag resistance, is directly influenced by the distribution of precipitates. It increases and then decreases with tempering temperature, after reaching its maximum value at 350°C. The additions of Mo and/or W result in a finer distribution of tempered carbide particles due to the decrease in the rate of the spheroidization and of the coarsening of carbide particles above 400°C. However, they do not affect the behavior of tempered carbides at low tempering temperatures. Accordingly, the additions of Mo and/or W increase the hysteresis loop area at tempering temperature above 400°C. The V addition increases the hysteresis loop area as well as hardness due to the precipitation of vanadium carbonitrides, regardless of tempering temperature. In considering industrial application of spring steels, a new parameter, the ratio of the hysteresis loop area in the torsional Bauschinger test to hardness, is suggested.

Microstructural influence on sag resistance of Cr containing and Cr free spring steels

The effects of Cr addition and tempering temperature on mechanical properties, especially sag resistance, with respect to the microstructural evolution during tempering in 1.5%Si spring steels, were investigated by performing hardness tests, tensile tests, torsional Bauschinger tests, and examination by transmission electron microscopy. The results obtained showed that the sag resistance depended on the distribution of tempered carbides. With increasing tempering temperature, sag resistance increased to a maximum value at 350°C and then decreased. The Cr addition did not affect the behaviour of tempered carbides at low tempering temperatures, while at high temperatures, above 400°C, the Cr addition reduced the coarsening rate of the cementite, resulting in finer particle distribution of the cementite. The Cr addition improved the sag resistance as well as the hardness at tempering temperatures above 400° C, but it decreased the ratio of loop area in the torsional Bauschinger test (the sag resistance) to hardness, regardless of tempering temperature.

Low-temperature ion nitriding used for improving the durability of the steel knives in the wood rotary peeling

In the production of veneer, the knives used in the rotary cutting of wet wood at a speed of about 1–2 m s −1 are made of low-alloy steels. These steels are used because of economical reasons, but, since they have a low tempering temperature (up to 300 °C), problems arise in treating their surface by the CVD and PVD methods which, to ensure the desired adhesion of the layers, require that the temperature of the substrate should exceed 500 °C. In view of the high friction coefficient of wet wood in contact with steel, the anti-abrasive layers must have an excellent adhesion and should be sufficiently thick. These are properties that can be difficult to obtain in combination at temperatures lower than 500 °C. The present study investigates low-temperature ion nitriding of 60SMD8 steel cutting knives. The service life of these nitrided knives, when used for the peeling of beech wood, increased by 100%.

Bainite Formation in High Carbon Copper Bearing Steels

The bainite reaction has been studied extensively in steels and other non-ferrous alloys. However up to present a significant disagreement exists regarding the nature and the mechanism of this phase transformation. During the isothermal bainitic transformation in hypereutectoid carbon steels alloyed with copper or copper and nickel, it was found that at all the transformation temperatures studied, the formation of equally copper supersaturated bainitic ferrite and cementite always occurred. This observation implies the formation of bainitic ferrite and cementite from the parent austenite phase without redistribution of the alloying elements, since the solubility of copper in cementite is negligible and very low in bainitic ferrite. By carefully designing suitable tempering treatments, it is possible to produce copper precipitation within tempered bainitic ferrite in both types of steels using low tempering temperatures. In addition, copper precipitation within the tempered bainitic cementite of the copper steel has been found to occur within a narrow tempering temperature region. However, the orientation relationship of the copper precipitation, within the tempered bainitic cementite and within the tempered bainitic ferrite has been found to be different. The interpretation of these experimental data strongly supports the theory promoting formation of the bainite through a shear controlled type of mechanism.

Effect of copper additions on the isothermal bainitic transformation in hypereutectoid copper and copper-nickel steels

During the isothermal bainitic transformation in hypereutectoid steels alloyed with copper or copper and nickel, it was found that at all the transformation temperatures studied, the formation of equally copper supersaturated bainitic ferrite and cementite always occurred. This observation implies the formation of bainitic ferrite and cementite from the parent austenite phase without redistribution of the alloying elements, since the solubility of copper in cementite is negligible and very low in bainitic ferrite. By carefully designing suitable tempering treatments, it is possible to produce copper precipitation not only within tempered bainitic ferrite in both types of steel using low tempering temperatures, but also within the tempered bainitic cementite of the copper steel at higher tempering temperatures. The interpretation of these experimental data strongly supports the theory that bainite formation is promoted through a shear controlled type of mechanism.

Novel applications of CrN (PVD) coatings deposited at 200 °C

CrN (PVD) coatings, deposited at 200 °C in plasma beam sputtering apparatus, were used for selected applications in industry. Using CrN coating, we improved two types of tool: moulds for Al alloy die casting (made of AISI H11 tool steel) and cold forming tools (made of AISI D2 and D3 tool steel). For the first group of tools, it was important that the CrN coating could be successfully used as a hard, oxidation- and corrosion-resistant coating at working temperatures of up to 800 °C; for the second group of tools, the low tempering temperature dictates deposition at 200 °C. The tool life, surface quality of the products and wear mechanisms were studied for the following polished tools (max. diameter, 210 mm): a mould for Al-Si12 die casting of small components of compressors; a combined cold forming and cutting die for mass production of covers for the heating plate of an electric stove; a cold forming deep drawing die for the production of a halogen lamp housing in the electric light industry; a tool set for automatic gradual deep drawing of fancy goods for the shoe and leather industry; a deep drawing die for mass production of the covers of electric motors. All the results show that a CrN coating 4.5–5.5 μm thick with a Cr intermediate layer 0.2 μm thick between the substrate and the coating improved the technological processes and resulted in a noticeable improvement in performance tests. Increased tool life, a higher quality of surface finish of the products and high reliability of CrN coated tools in production were regularly observed during long-term performance tests.

Mechanical properties of 30KhMA steel after quenching and low tempering with preheating in the intercritical temperature range or toughening

Mechanical properties of 30KhMA steel after quenching and low tempering with preheating in the intercritical temperature range or toughening

Disposition of hot-formed structural steels with 0.4% C to temper brittleness

1. Obtaining a polygonized structure in hot forming of austenite in a cycle of HTMT of steels 40Kh, 40KhN, and 40KhN2MA causes an increase of the resistance to brittle failure: after low tempering and embrittling tempering at 500°C (for 200 h) the level of impact toughness increases at normal as well as subzero temperatures; the critical brittle point is lowered; the fraction of crystalline component in the fracture decreases. 2. Hot forming of structural steels with 0.4% C at the steady state of dynamic polygonization (e≌50%) even with partial development of dynamic recrystallization (up to 30% vol. %) is more preferred for reducing disposition to temper brittleness than at the stage of nonsteady dynamic polygonization (e≌25%). Obtaining fully dynamically recrystallized austenite with finer grain than the initial one reduces to a lesser extent the disposition of steels to temper brittleness. 3. The lowering of the critical brittle point T20 as a result of HTMT of steels is directly proportional to the decrease of the fraction of intergranular failure.

Causes of cleavage on case-hardened gear wheels

1. It was established by methods of macrofractography and microfractography that the typical defects of case-hardened gear wheels, viz., cleavage, are brittle hindered failure originating in the transition zone of the case and spreading from the central part of the fracture. 2. The principal cause of cleavage are residual tensile stresses which arise as a result of thermochemical treatment on account of incorrect choice or infringement of the regimes of heat treatment and thermochemical treatment, and also as a results of excessive work-hardening by shot-peening. 3. Cleavage is promoted by factors enhancing the proneness of steel to hindered failure and increasing the residual tensile stresses in the transition zone of the case: proneness to grain growth and overheating, supersaturation of the case and of the corner formed by three sides of the both with carbon, high cooling rate in quenching, too low a temperature and insufficiently long holding in low tempering after quenching. 4. Measures for preventing cleavage in case-hardened gear wheels have to aim at reducing the proneness of steel to hindered failure and at ensuring favorable curves of the distribution of residual stresses arising as a result of thermochemical treatment and subsequent workhardening by shot-peening.

Effect of production factors on the ultimate static-bending strength of carburized steels

A reliable and labor-free method of evaluating treatment factors on the properties of carburized steels was developed. The effects of the following parameters are presented along with treatment factors for steel 18Kh2N4MA and 20Kh2N4A: austenite grain size, hydrogen absorption during heating for quenching, cooling rate, quenching methods, and low tempering. Results of the comparative static bend tests made it possible to recommend the new steels 07Kh3GNM and 20Kh3MVFA in lieu of chromium-nickel steels because of their improved machinability, weldability, and reduced deformation. Steel 20Kh2N4A was subjected to a new carburizing process consisting of ion and vacuum carburizing which was found to increase strength and optimize carbon content.

Factors determining the modulus of rupture of biphase ferritic-martensitic steels

1. An increase of the modulus of rupture of BFMS under dynamic loading can be attained by reducing the strength of martensite (its carbon content) and comminuting its sections, which is achieved by reducing the concentration of carbon in the steel, by hardening at a higher temperature of the biphase region, and by preparing the corresponding initial structure. 2. Comminution of the ferrite grain in BFMS and reduction of the distortions of the α-lattice by interstitial atoms by reducing the cooling rate, the introduction of carbonitride forming elements, or the use of concluding low tempering (200–250°C) make it possible, with greater strength of the BFMS, to ensure better fracture characteristics than in steels with the same composition but with ferritic-pearlitic structure. 3. A characteristic trait of BFMS is the predominant crack development in one of the phases: the sections of ductile failure on the surface of impact test specimens tested in the range of the brittle-ductile transition belong basically to the martensitic phase; the fatigue crack, especially in the region of small ΔK, develops predominantly in the ferritic matrix.

Residual stress effects during near-threshold fatigue crack growth

The fatigue crack propagation behaviour of a low alloy, boron-containing steel has been examined after austenitizing at 900°C or 1250°C and tempering at a range of temperatures up to 400°C. Fatigue threshold values were found to vary with austenitizing and tempering treatment in a range between 3.3 to 6 MPa √m when tested at a stress ratio ( R) of 0.2. Crack propagation rates in the Paris regime were insensitive to heat treatment variations. The crack propagation path was essentially transgranular in all conditions with small regions of intergranular facets appearing at growth rates around the knee of the d a/d N vs Δ K curve. The crack front shape showed marked retardation in the centre of the specimen at low tempering temperatures. Experimental determinations and computer predictions of residual stress levels in the specimens indicated that this was due to a central residual compressive stress resulting from differential cooling rates and the volume change associated with the martensite transformation. The results are discussed in terms of microstructural and residual stress effects on fatigue behaviour.

Structure and contact endurance of case-hardened steel 12Kh2N4A after plastic surface deformation

1. Improved contact endurance of case-hardened steel 12Kh2N4A by shot-peening is attained in the narrow range of strain-hardening regimes in which the substructure of the martensite of the case does not suffer local damage. 2. It is recommended to combine plastic surface deformation with subsequent low tempering that is necessary for the relaxation of local microstresses, and also for additional strengthening of the martensite by strain-aging. As a result of such combined hardening the contact endurance of steel 12Kh2N4A increases threefold.

Effect of the tempering conditions on the quality of patented wire

1. The quality of wire for HPH can be evaluated from the generalized desirability function determined with the aid of the partial desirabilities of the rupture load of the wire without knot and with knot, and the number of twists to failure. 2. The dependence of the generalized desirability function on the tempering conditions, obtained from the results of the investigation of the effect of the temperature and time of tempering on the mechanical properties of wire RM-S-IB of the Cheropovets Steel Rolling Plant, has the following form: D=0.0074666+0.00659τ+0.015545τ−0.000053tτ−0.000013t2 (t=150 ... 450°C, τ=2 ... 22 min. 3. From the obtained diagram of the dependence of the generalized desirability function on the tempering conditions we can determine the conditions ensuring the level of "very good" quality of the wire (D up to 0.86). 4. To improve the quality of wire and reduce the amount of scrap, we must introduce low tempering as the concluding operation of the production process of the wire.

Application of Chemical and Physical Methods for the Reduction of Tool Wear in Bulk Metal Forming Processes

Summary Tool wear reduction in bulk metal forming by applying various types of surface coatings was investigated by two forming processes “backward extrusion of cans” and “upsetting between flat parallel dies”. Nitrided layers and hard chromium plating was tested as well as modern coatings like PVD, CVD or ion implantation. To reduce the effort of tests a short time testing procedure based on radionuclide technique was developed, where a thin surface layer of the tool has to be activated by using a cyclotron. During the wear process the activity of the tool decreases due to the loss of material. The difference of activity is a measure of the wear rate. By testing nitrided punches during backward extrusion, cracks in the nitrided layer appeared in some cases. If cracks do not appear, the amount of wear of the nitrided punches decreases to 25 - 65 % of the value for uncoated punches. The best results were obtained for PVD and TD coated punches. This agrees with experience made in the industry. By applying ion implanted tools, wear decreases down to the amount of nitrided tools. The great advantage of this process is the temperature of treatment below 200 °C. Therefore roughness and structure of material are not affected. There is a possibility of treating steels which have a low tempering temperature, and tools can be machined to their final dimensions before ion implantation. In upsetting, both CVD-TiC and hard chromium coatings showed good wear resistance. The comparison of both forming processes, different tool materials, workpiece materials and layers should contribute to an economic application of optimized wear resistant tools in bulk metal forming processes.

Optimization of the heat-treatment parameters for castings of steel 23KhGS2MFL

This article contains the results of the investigation on the effect of the heating medium on the proneness of steel to scaling and decarburization, and of determining the optimum heat-treatment regime. The properties of tapered samples of steel 23KhGS2MFL from whose lower part specimens 15 x 15 x 200 mm in size were cut out, were investigated. The effect of the quenching temperature on the mechanical properties of the specimens after final heat treatment were determined. The regime suggested for heat treatment for castings of steel 23KhGS2MFL is high tempering at 740-750/sup 0/C, quenching in water at 1020/sup 0/C (1 h) and subsequent low tempering to the specified hardness level. The introduction of this suggested regime of heat treatment ensures that mechanical properties according to GOST 977-75, hardness according to the technical conditions, and the permissable thickness of the decarburized layer are attained, even when the metal is heated without a controlled atmosphere.

Effect of aging of supercooled austenite on the mechanical properties of vanadium-containing steels

1. The development of the processes of dispersion hardening of supercooled austenite leads to improved strength properties of steels 30Kh3N3MFB and 30KhNSMFB (with a slight reduction of ductility and real fracture stress) after hardening with isothermal holding in the range of subcritical temperatures and low tempering compared with the properties of these steels obtained by conventional hardening and the same tempering. 2. The uniform distribution of regions enriched with vanadium and carbon forming in the process of isothermal transformation of supercooled austenite and inherited by martensite, the possibility of their being cut by dislocations cause the uniform development of plastic deformation throughout the bulk of the steel without stress concentrations arising. Such a structural state ensures the possibility of attaining good strength properties. Then failure of steels 30Kh3N3MFB and 30KhNSMFB proceeds by the ductile micromechanism, but when the isothermal holding before quenching is extended to more than 10 min, the tendency of steel 30KhNSMFB to brittle failure increases; this may be due to the effect of silicon. 3. The good strength properties combined with satisfactory ductility and real fracture stress enable us to recommend the newly devised steels, especially 30Kh3N3MFB, as material for improtant machine parts.

The role of nitrogen in the embrittlement of steel

Nitrogen is one of the most common impurity elements to be found in steels. Previous work has shown that it is a potential grain boundary embrittler. In this paper we examine its role in both tempered martensite embrittlement and temper embrittlement. The basic composition of the steel used for this study was, in wt pct, 3.5 Ni, 1.7 Cr, 0.3 C, and 0.01 N. It was found that nitrogen could be very detrimental to mechanical properties but not as a grain boundary embrittler in the typical sense that P, Sn, and Sb are. Rather, nitrogen is almost always precipitated as nitrides and these second phase particles can induce low energy ductile fracture. The distribution of nitrides in the solid and the type of nitride present is dependent on the heat treatment. If a low austenitizing temperature is used, the nitrides in the steel dissolve and considerable nitrogen segregates to the grain boundaries. During an oil quench it reprecipitates at the boundaries, primarily as Cr2N. These nitrides cause low energy, ductile intergranular fracture. If a high austenitizing temperature is used, much less nitrogen segregrates so fewer nitrides precipitate during the quench. However, upon tempering the nitrogen does reprecipitate. At low tempering temperatures, small nitrides form both within the grains and along the grain boundaries. When these nitrides become sufficiently large, voids form around them as well as around the carbides during fracture. These small voids help link the large voids that form around oxide and sulfide particles and lower the energy for ductile fracture. After high temperature tempering treatments large nitrides and carbides form at the grain boundaries. These produce low energy, intergranular ductile fracture. These large grain boundary precipitates can also aid in brittle intergranular fracture by providing many more sites for nucleation of intergranular cracks when the boundary is weakened by another impurity element.

Use of high-frequency-current hardening and surface plastic deformation to increase contact endurance

1. Resultant equations (3) and (4) can be used, with certain approximation (modeling by the \ldroller analogy\rd method), to compute allowable stresses for the design of steel 40Kh contact pairs, which are subjected to HFC hardening with low tempering and hardening rolling and which operate under rolling conditions with minor slippage. 2. For the tool, used in our study, we recommend that rolling be conducted in a single pass with the smallest axial longitudinal feed possible. When other generating rollers are used, it is necessary to select the load, considering that the optimum load in the present investigation was P=1200 N for which the maximum normal contact stress from the Hertz-Belyaev equations σe = (3P/2ab)(1/3) = 2600 MPa. On rolling with a single-roll device the number of passes can be increased. 3. The method used in the present study is recommended for the development of an optimum hardening regime for the rolling of components to ensure maximum fatigue strength, corrosion resistance, sensitivity to stress concentrators, etc. In these cases, other mathematical, models of the function Q should be devised for each specific case with allowance for specific features of the characteristic under investigation. 4. Hardness exerts the greatest influence on contact endurance, while internal stresses, and microgeometry are of secondary importance.

Effect of reinforcement on the structure and properties of steel wire

1. Reinforcing mild steel with fibers of temper-hardened steel improves the quality of wire rod. Hot-rolled wire rod was successfully subjected to cold drawing to a 3.0-mm diameter without heat treatment. 2. The mechanical properties of reinforced wire can be improved by heat treatment. For the composite steel 0.8kp-steel 40, good strength and ductility were obtained after hardening at 860 °C and low tempering. 3. Reinforced wire rod has higher strength than predicted by the rule of additivity. A large contribution to the higher strength of reinforced specimens is made by the structure of the composite, which becomes more complex during solidification, subsequent plastic strain, and heat treatment.