Double Tempering Research Articles

The effect of cryogenic treatment on a high Co bearing DIN 1.2888 steel used as die in hot forging process

Abstract Hot work steels used as dies in metal forming processes must endure harsh conditions, typically achieved through a martensitic structure. However, retained austenite can persist after martensitic transformation, negatively impacting mechanical properties. Cryogenic treatment (CT) refines martensite, reduces retained austenite, and increases fine carbide density, thus enhancing performance. DIN 1.2888, a hot work tool steel with high cobalt content (~10%) and low carbon content (~0.2%), is less studied concerning the effects of CT. The objective of this study was to investigate how cryogenic treatment affects the mechanical and microstructural properties of DIN 1.2888 steel. Samples underwent conventional heat treatment (HT) and CT at -100, -140, and -180°C for 6 hours, followed by double tempering. Various analytical methods, including X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and Vickers Microhardness tests, were used to examine the samples. Wear mechanisms were evaluated through pin-on-disc tests under different loads, and impact toughness was measured both at room temperature and the working temperature of dies (350°C). Results indicated that lowering the cryogenic treatment temperature resulted in a slight increase in hardness values from 507HV for the heat-treated sample to 529HV for the sample treated at -180°C. Additionally, the impact toughness improved significantly, with values rising from 12.35J for the heat-treated sample to 23.44J at 350°C for the cryogenically treated sample at -180°C. Wear rates also decreased by approximately 50%, particularly at higher cryogenic treatment temperatures. The improvement in wear resistance was attributed to finer carbide precipitation and a more homogeneous distribution of carbides. These findings suggest that deep cryogenic treatment has a positive effect on DIN 1.2888 steel by decreasing retained austenite and increasing fine carbide density, leading to enhanced mechanical properties and extending the lifespan of the steel in die applications.

Influence of cryogenic treatment and tempering temperature on microstructural evolution and dry sliding wear behavior of AISI D3 cold-work tool steel

Abstract AISI D3 cold work steel was shallow and deep cryogenically treated and double tempered at 150, 250, and 350 °C temperatures. Cryogenic processes transformed the retained austenite into martensite, while double tempering produced Fe, Cr, and W-rich carbides. The wear losses of cryogenically treated specimens decreased by up to 60% compared to conventionally heat-treated samples. Worn surfaces mainly experienced abrasive and adhesive wear mechanisms. Due to the formation of homogeneously dispersed fine carbides at 250 °C, oxidative wear occurred at the matrix phase, resulting in the lowest wear rate. The samples tempered at 150 °C suffered from the severe abrasive action of hard carbide particles, while the samples treated at 350 °C failed because of carbide coarsening.

Enhanced Durability of Wood Cutting Tools through Thermal Cycling.

This study investigates the impact of multi-step austenitization heat treatment on the in-service life of modified AISI A8 cold work tool steel knives used in wood cutting. The knives were subjected to two treatment methods: single quenching and double tempering (SQDT) and double quenching and double tempering (DQDT). Both treatments were followed by physical vapor deposition (PVD) coating to enhance surface properties. The DQDT treatment resulted in a finer microstructure and more uniform carbide distribution. Field tests on 24 knives over 124 h demonstrated up to 130% improvement in wear resistance for DQDT knives, along with superior edge stability and better PVD coating preservation. DQDT knives exhibited ductile fractures characterized by dimples, contrasting with the brittle fracture and cleavage facets in SQDT knives. Residual stress measurements showed higher compressive stresses in DQDT knives (-280 MPa) compared to SQDT knives (-30 MPa), which increased further after field testing. The enhanced performance of DQDT knives is attributed to their refined microstructure, improved carbide distribution, and higher compressive residual stresses, offering significant potential for improving wood cutting tool efficiency and durability.

The Effect of Cryogenic Treatment on W360 and E38K Hot Work Tool Steels

The present study examines the impact of cryogenic treatment (CT) on W360 and E38K hot work tool steels used in forging dies, subjected to conventional heat (HT) and CTs followed by double tempering treatment. Results reveal that CT facilitates the transformation of retained austenite from 4.9% and 2.5% for W360 and E38K steels, respectively, into martensite to an undetectable limit. Also, when compared to HT, CT at −180 °C results in about 3 times higher amount of the carbide precipitation for both steel types. In contrast, a slight increment in the hardness values with decreasing treatment temperature is observed. Notch impact test results indicate higher impact energy with lower CT temperatures, particularly notable in W360 steel. Although both steel types exhibit almost similar impact energy values at room temperature, E38K steel has ≈100% higher energy at 350 °C. DCT‐treated samples show improved wear resistance at lower cryogenic temperatures. Cryotreated W360 steel displays nearly 25% less wear rate than cryotreated E38K steel. The study underscores the suitability of W360 and E38K steels for hot work applications, with CT enhancing their mechanical and microstructural performance, particularly in W360 steel due to its higher carbide density.

On the Effects of Deep Cryogenic Treatment on Wear Resistance, Hardness and Microstructure of the AISI D2 and D3 Tool Steel

The present study investigates the effect of cryogenic treatment on AISI D2 and D3 tool steels. This analysis is carried out for several heat treatment cycles that follow various tempering and cryoprocessing sequences that are not analysed erstwhile. Moreover, the effect of these heat treatment cycles on wear characteristics, hardness, and microstructural features of AISI D2 and D3 tool steel is also analysed. It is demonstrated that the cryogenic treatment significantly improves the wear resistance, hardness, and retained austenitic transformation of both types of material specimens under consideration. Furthermore, it is established that single tempering after cryogenic treatment is more effective than double tempering processes carried out in different sequences. It is also revealed that the double tempering sequencing involving cryoprocessing stalls the transformation of retained austenite, leading to higher wear and low hardness in the material

General investigations on the heat treatment and thermal fatigue behavior of an experimental hot work tool steel tailored for laser powder bed fusion

An experimental hot work tool steel with a leaner chemistry compared to the standard AISI-H13 grade, aimed at enhanced processability, was gas atomized and processed using laser powder bed fusion (L-PBF) to >99.8 % relative density. The lower carbon content (∼0.25 wt% vs. ∼0.4 wt% in H13) resulted in a softer as-built (AB), and quenched martensite (∼485 HV1 vs. 625 HV1 in H13), higher martensite-start temperature (∼360 °C vs. 280–320 °C), and about ∼18 % reduction in dilatation, ascribed to the volume expansive martensitic transformation. Charpy V-notch impact toughness in AB condition (>30 J) was higher than those reported for AB H13. These features account for the improved L-PBF processability. The tempering response in comparison with a wrought H13, was characterized by a larger precipitation of thermally stable Mo- and V-rich secondary hardening alloy carbides at the expense of the easy-to-coarsen Cr-rich ones, attributed to the higher Mo and lower Cr content in experimental alloy. The combination of lower C- and V-contents in experimental alloy, reduced the driving force for the precipitation of coarse vanadium rich carbides during austenitization, as confirmed by electron diffraction spectroscopy (EDS) and synchrotron X ray diffraction. This led to a stronger fine secondary hardening alloy carbide precipitation during tempering, and enhanced temper resistance. As elaborated by isochronal tempering, as well as tempering curves, reduction of the elements Si, and Cr shifted the secondary hardness peak to higher tempering temperatures. Consequently, tempering, and thermomechanical softening resistance was significantly enhanced in the new steel with a maximum hardness of ∼550 HV1 in over-tempered condition (i.e., slightly above secondary hardness peak). Thermal fatigue (TF) tests revealed a denser and finer TF crack network with a larger oxidation in experimental steel. Thereby, the TF crack penetration depth scaling with the local plastic yielding of the matrix was evidently shorter in the new steel compared with wrought H13, thanks to significantly improved hot hardness, thermal conductivity, and enhanced thermomechanical softening resistance. Finally, a proof of concept is demonstrated by processing a relatively large injection molding die (150 mm × 110 mm × 30 mm) with promising tensile and impact toughness properties after direct double tempering to 51 HRC, and 45 HRC hardness levels.

Failure Analysis of Cryogenically Treated and Gas Nitrided Die Steel in Rotating Bending Fatigue

<div>AISI H13 hot work tool steel is commonly used for applications such as hot forging and hot extrusion in mechanical working operations that face thermal and mechanical stress fluctuations, leading to premature failures. Cryogenic treatment was applied for AISI H13 steel to improve the surface hardness and thereby fatigue resistance. This work involves failure analysis of H13 steel specimens subjected to cryogenic treatment and gas nitriding. The specimens were heated to 1020°C, oil quenched followed by double tempering at 550°C for 2 h, and subsequently, deep cryogenically treated at −185°C in the cryochamber. Gas nitriding was carried out for 24 h at 500°C for 200 μm case depth in NH<sub>3</sub> surroundings. The specimens were subjected to rotating bending fatigue at constant amplitude loading at room temperature. Measurement of surface roughness, hardness, and microstructural analysis indicated improved fatigue life for cryogenically treated specimens as compared to gas nitride, which could be attributed to fine carbide precipitation accountable for the delayed crack initiation and propagation.</div>

Achieving ultrahigh yield strength and decent toughness in an in-situ alloyed H13 steel via laser powder bed fusion

It is highly challenging to simultaneously achieve ultrahigh yield strength (YS) and decent toughness in the H13 steel via laser powder bed fusion (L-PBF). In this contribution, a double tempering strategy is used to optimize the microstructure and mechanical performance of an in-situ alloyed H13 steel fabricated via L-PBF. The primary tempering can result in the formation of fresh martensite and the relatively unstable retained austenite (RA), which substantially deteriorates impact toughness and yield strength (YS). The appropriate double tempering can effectively eliminate fresh martensite and reduce the fraction of the unstable RA to below 5%, successfully realizing decent mechanical properties with an ultrahigh YS of ∼1.8 GPa, an ultimate tensile strength (UTS) of ∼2.05 GPa, and a Charpy V-notch (CVN) value of 17 J in an in-situ alloyed H13 steel via L-PBF, which is comparable with those of the wrought H13 steel. The current study demonstrates that the elimination of fresh martensite and optimization of austenite stability are the key to improve the mechanical performance of L-PBF ultra-high strength steels with medium carbon content.

Increased Hardness Value of Medium Manganese Steel Through Double Tempering, Hot Rolling, and Variation of Cooling Media

Research has been conducted to enhance the hardness value of medium manganese steel through a heat treatment. Initially, this process begins with austenization at a temperature of 900°C, followed by tempering at 650°C and double tempering at 600°C, with each stage lasting 30 minutes. Subsequently, each stage concludes with a hot rolling process, after which air or water cools the material. As a result of these processes, the hardness tests revealed an increase in the hardness of medium manganese steel, reaching up to 389.70 BHN with a tensile strength of 827 MPa, which was notably achieved through air cooling. This significant increase in hardness is attributed to the emergence of the martensite phase and the presence of a large number of carbides, which are more evenly distributed after the double-tempering process. Additionally, small amounts of carbides were observed in the austenite matrix. Upon examination of the SEM fractography results, it was revealed that the fracture was mixed, with a cleavage area slightly larger than the dimple area. This observation suggests that despite its high hardness value, the sample retains good toughness.

Features of hydrogen cracking process of heat-strengthened reinforcing steels

The effect of heat treatment of reinforcing steels of St5, 18GS, 20GS, 23Kh2G2Ts, 30GST and 35GS grades on the change in their resistance to hydrogen cracking is considered. For St5 steel, it was found that an increase in the tempering temperature contributes to an increase in resistance to cracking by obtaining a highly plastic structure with an increased ability to relax internal microstresses. For 30GST and 35GS steels, optimal heat treatment modes are determined to ensure their resistance to hydrogen cracking. For 30GST steel, this mode is quenching from 900 °C with cooling in water + tempering with electrical heating at 500 °C, and for 35GS steel — triple heat treatment of the type of intercritical quenching from temperatures of 900/700 °C or 800/700 °C, followed by tempering at temperatures of 300 or 400 °C, respectively and heat treatment from 900 °C with double tempering (from 400 °C (furnace heating) and 500 °C (electrothermal hardening)). The features of the destruction process of reinforcing steels in an environment causing hydrogen cracking are considered. It is shown that the propagation of corrosion cracks has a mixed character (transcrystalline and intercrystallite).

The effect of quenching/double quenching and tempering heat treatment cycles on the microstructure, impact energy and hardness of AISI H13 tool steel

ABSTRACT In this study, microstructure modifications of AISI H13 steel were studied with different heat treatments such as conventional quenching, quenching + tempering/double tempering and double quenching + tempering/double tempering. Carbide volume ratio analyses were performed. The XRD analysis was used to determine the type of carbide, the average crystallite size, and the dislocation density in the samples. Charpy impact tests were performed. The hardness values of the samples were measured with the Rockwell C scale. The presence of MC, M23C6, and M7C3 type carbides was determined in the microstructures. Heat treatment cycles caused a high dislocation density in martensite laths. The dislocation density of martensite laths decreased with double quenching. The tempering process caused the formation of fine carbide particles in the microstructure. In addition, tempering processes gave rise to an increase in the volume ratio of M23C6-type strip carbides. The length/width ratio of the strip-type carbides increased with the effect of double tempering. As a result of the applied heat treatments, the impact energies of the samples decreased. The tempering process caused a secondary hardening of the samples. While the double quenching process increased the impact energy value by approximately 2%, it caused a decrease in hardness by 24%.

Characterization of the mechanical properties and microstructural evolution of martensitic steel in repeated tempering cycles

The purpose of this study was to understand the behavior of martensitic H13 steel in accordance with the microstructural evolution, mechanical properties and wear in repeated tempering cycles. The microstructures were characterized by axio image observer microscope, scanning electron microscope (SEM), x-ray diffraction (XRD). Uniaxial tensile test, charpy v-notch impact test, rockwell hardness test and wear test were conducted to analyze the changes in mechanical properties, impact properties, hardness and wear in repeated tempering cycles. The specimen prepared were subjected to the hardening at 1030 °C for 20 minutes, oil quenched and subjected to repeated tempering cycles at 570 °C for 2hrs holding time each. The mechanical properties recorded indicates that the maximum ultimate tensile strength obtained was at double tempering due to secondary hardening effect i.e., alloy carbides precipitation offering strength to the matrix and corresponding wear was found to be minimum. The annealed specimen revealed bainitic microstructure and with hardening and repeated tempering cycles, fine needle like structure and carbides was observed in microstructure and retained austenite was converted into martensite and martensite was converted into tempered martensite. Carbide size and martensite lath distribution controls the strength and fracture rate.

Influence of a Novel Double Tempering Process on the Microstructure and Mechanical Properties of Cu-Alloyed Austempered Ductile Iron with Possible Nano (Micro)-Characterization Using Neutron Beam Techniques

In this paper, a novel method for the double heat treatment of ductile iron was applied. Ten sets of specimens (three specimens in each set) of ductile cast iron (DCI) containing 0.51% wt. Cu were prepared and converted to austenitic ductile iron. All specimens were austenitized at 850 °C for 60 min and annealed at 420 °C, 331 °C and 250 °C for 120, 68 and 30 min, respectively. Five sets of samples were then annealed at 500 °C for 60 min, creating a novel double heat treatment process for annealing. Finally, all specimens were slowly cooled in air at ambient temperature. Tensile strength, hardness and elongation were measured in all specimens to compare the specimens with and without subsequent tempering. A microstructural analysis was also performed, which showed that the microstructure changed for the specimens that were subsequently tempered with. The results show that specimens with subsequent tempering have slightly higher hardness, a small decrease in tensile strength and significantly higher elongation. In addition, specimens with subsequent tempering exhibit more uniform mechanical properties compared to specimens without subsequent tempering. The use of neutron beam techniques was proposed to further characterize the newly formed microstructure after subsequent tempering.

Sliding wear behaviour of conventional and cryotreated PM Cr-V (Vanadis 6) ledeburitic tool steel

Cryogenic treatment is a widely accepted method for extending the tool life of tool steels. This study focuses on investigating the tribological properties of PM Cr-V cold work tool steel subjected to conventional and cryogenic treatments. The aim of the study is to compare the reciprocal sliding wear performance of Cr-V cold work tool steel under different treatment conditions. The Cr-V tool steel specimens were cryotreated at temperatures of −75 °C, −140 °C, or −196 °C, followed by double tempering at 170 °C or 530 °C for 2 h. Cryotreated specimens exhibited a 10% increase in hardness compared to conventionally treated specimens after tempering at 170 °C. However, specimens cryotreated and tempered at 530 °C showed a 7% drop in hardness. Reciprocating sliding wear tests were conducted at room temperature using an Al2O3 ball as the counter-body to simulate abrasive wear. The results indicate that the friction coefficient is primarily influenced by the applied load and sliding velocity rather than the material treatment. Cryogenic treatment at 170 °C improved the wear performance by approximately 24% compared to conventionally treated specimens, while cryotreated specimens tempered at 530 °C did not exhibit any improvement in wear performance compared to conventionally treated specimens.

Influence of the heat treatment on the mechanical and tribological properties of cryogenically treated AISI M2 steel

Cryogenic treatments are known to increase the wear resistance and toughness of steel. However, the literature reports some contradictory results. This work aims to investigate and determine the optimum cryogenic parameters combined with cycles of quenching and tempering of AISI M2 tool steel to increase wear resistance and toughness. Charpy specimens were austenitized (1185 °C, 1200 °C or 1215 °C), quenched to room temperature, cryogenically treated to −196 °C (20 h or 28 h), and subjected to double tempering (520 °C, 535 °C or 550 °C). A Taguchi L9 DoE approach was used to determine the optimum set of heat treatment cycles. Mechanical and tribological properties were investigated in terms of wear, impact, hardness, and microhardness tests. The results show that austenitization temperature (AT), tempering temperature (TT), and cryogenic treatment (CT) time are interdependent. Depending on the AT and TT parameters used, CT can improve, have little influence, or even deteriorate the mechanical and tribological properties of AISI M2 steel. The amount of retained austenite after CT ranged from 4.3 to 7.6%. Cryogenic treatment improves wear resistance and impact toughness simultaneously.

Evaluation of post weld heat treatments and susceptibility to sulfide stress corrosion cracking of simulated HAZ in forged supermartensitic stainless steel UNS S41427

Supermartensitic stainless steels (SSMSs) with 12–13%Cr, 5%Ni, 2%Mo and microadditions were developed to casings, tubullars and mandrels used in the oil and gas off-shore production. These materials have great resistance to aqueous medium containing large amounts of CO2 and can withstand high temperatures and large amounts of chloride as well. However, SMSSs have a limited resistance to H2S containing solutions, and may fail due to sulfide stress cracking (SSC). This limit is even lower for forged and welded components that used in mandrels. Recent studies have been developed to find an operational envelope for safe use of SMSSs in sour production systems. The present study shows the results of slow strain rate testing (SSRT) of a forged V-alloyed SMSS submited to thermal simulation in Gleeble machine to produce a coarse grain heat affected zone (HAZ). The microstructure of the HAZ is characterized by fresh martensite, intergranular delta ferrite and fine precipitates which increase the hardness to a level above the maximum limit for sulfide stress corrosion cracking. The effects of two post weld heat treatments (PWHT) on the mechanical properties and susceptibility SSC were evaluated. Single and double tempering treatments provoked the decrease of hardness and the increase of ductility parameters. However, specimens representative of the HAZ, base metal, and HAZ with single and double tempering presented high SSC susceptibility parameters in SSRT in saline solution (200,000 ppm NaCl) with 1.0 psia of H2S partial pressure, and pH 4.5.

Effect of double tempering process on sulfide stress cracking susceptibility in API-5CT-C110 casing steel

The sulfide stress cracking (SSC) of the API-5CT-C110 grade casing steel in this work was evaluated by double cantilever beam (DCB) testing, two thermal treatments were used to investigate microstructures and properties. Compared to one, long-term tempering treatment, the study showed that precipitate size and dislocation density significantly decreased if two, short-term tempering treatments were used instead. DCB testing showed that SSC resistance is improved by double tempering with no degradation of strength. Specimens treated by double tempering have a lower proportion of Σ3 boundary, high-angle grain boundaries and high Taylor factor values, reduced susceptibility to crack initiation and propagation.

Experimental and numerical analyses of residual stress redistributions in large steel dies: Influence of tempering cycles and rough milling

The aim of the present research was to study the residual stress variations induced by heat treatment and rough milling operations of large forged steel blocks. The heat treatment process includes quenching and tempering operations. An efficient strategy based on a size sensitivity analysis (SSA) was proposed to reach the actual values of residual stresses in large-size steel blocks by using small-size workpieces in laboratory scale without damaging the blocks. The residual stresses were measured and compared after the first and second tempering and the rough milling. A 3D finite element (FE) model was developed to predict superficial residual stresses and was then calibrated experimentally. The results showed that the residual stresses after the first and second tempering were highly compressive on and near the surface. In addition, the resultants of the residual stresses after double tempering processes were almost equal. Therefore, the second tempering could be omitted from the manufacturing process. It was also found that both surface and sub-surface hoop and radial residual stresses were highly tensile after the rough milling. Finally, the 3D FE model can be used as a predictive tool to predict residual stresses for rough milling to avoid conducting expensive, time-consuming experimentations and measurements.

Effect of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel

The effects of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel (13-4MSS) were investigated by experiments and crystal plasticity finite element modeling (CPFEM). The tempering structure mainly includes tempered martensite lath, reversed austenite and M23C6 carbide. The tempered martensite lath is refined by the tiny new martensite transformed by reversed austenite, and the refinement degree increases with the tempering temperature. The nucleation of reversed austenite depends on the new martensite and carbide generated during tempering, and more reversed austenite is obtained by double tempering. The cryogenic impact toughness of the material changes nonlinearly with the increase of tempering temperature. It indicates that fine martensitic lath and inverse austenite are beneficial to improve the cryogenic impact toughness of the material, while carbide particles play the opposite role. The results of CPFEM show that reversed austenite coordinates the impact force of the surrounding martensite through large plastic deformation, and eases the stress concentration. Reversed austenite even occurs DIMT behavior under large plastic deformation to further reduce the stress concentration and prevent the initiation and propagation of cracks. As a hard particle, carbide causes the stress concentration, leading to cracks initiation and propagation along the martensite boundary.

Regularities of changes in physical and mechanical and corrosion properties of heat-strengthened reinforcing 35GS steel

The effect of heat treatment on the change in the physical and mechanical and corrosion properties of reinforcing 35GS steel is considered. It is established that the use of triple heat treatment (intercritical hardening from temperatures of 900/700 °C or 800/700 °C, followed by tempering at temperatures of 300 or 400 °C, respectively, hardening from 900 °C with double tempering (from 400 °C (oven heating) and 500 °C (electro-thermal hardening)), significantly reduce the sensitivity of reinforcing 35GS steel to hydrogen cracking. During intercritical hardening of 35GS steel with obtaining properties corresponding to the At600 class during the first hardening from 900 °C, the second hardening from 700 °C and tempering up to 300 °C resistance in an environment causing hydrogen cracking at stress intensity factor of 0.6 increases by 10 times (19.5 hours versus 2 hours) and during the first hardening from 800 °C, the second hardening 700 °C and tempering up to 400 °C resistance increases by 20 times (40.5 hours versus 2 hours). It is shown that under conditions of intense hydrogenation, to reduce the sensitivity of steel to hydrogen cracking, it is more preferable to use volume rather than surface tempering.