Abstract Carburizing and induction hardening are two surface heat treatments commonly used to increase wear resistance and fatigue performance of steel parts subject to cyclical torsional loading. It was originally hypothesized that performing an induction surface hardening heat treatment on parts previously carburized could provide further increased fatigue life, however initial torsional fatigue results from previous work indicated the opposite as the as-carburized conditions exhibited better torsional fatigue strength than the carburized plus induction surface hardened conditions. The aim of this work is to further elucidate these torsional fatigue results through metallography and material property characterization, namely non-martensitic transformation product (NTMP) analysis, prior austenite grain size (PAGS) analysis, and residual stress vs depth analysis using x-ray diffraction (XRD). A carburizing heat treatment with a case depth of 1.0 or 1.5 mm and an induction hardening heat treatment with a case depth of 0, 2.0, or 3.0 mm were applied to torsional fatigue specimens of 4121 steel modified with 0.84 wt pct Cr. The carburized samples without further induction processing, the 0 mm induction case depth, served as a baseline for comparison. The as-received microstructure of the alloy was a combination of polygonal ferrite and upper bainite with area fractions of approximately 27% and 73% respectively. The only conditions that exhibited NMTP were the as-carburized conditions. These conditions also exhibited larger average PAGS and higher magnitude compressive residual stresses at the surface compared to the carburized plus induction hardened conditions. The compressive residual stresses offer the best explanation for the trends observed in the torsional fatigue results as the conditions with NMTP present and larger PAGS exhibited the best torsional fatigue performance, which is opposite of what has been observed in literature.

and the 31st Heat Treating Society Conference and Exposition was held in St Louis

Solid and hollow cylindrical specimens of ferrite–pearlite and austenitic stainless steel were induction heated at heating rates up to 600 °C·s −1 using a high precision DIL 805L dilatometer. High heating rates were attained for the magnetic state at temperatures below the curie ( A c2 ) temperature and reduced heating rates were observed for the non-magnetic austenite at temperatures above the A c2 . Heating the commonly used 4 mm diameter by 10 mm long specimens at rates above 50 °C·s −1 resulted in non-linear heating rates during transformation to austenite and large transient temperature variations along the specimen length. These limitations in heating rate and variances from ideal uniform heating lead to inaccurate measurement of the A c1 temperature. Induction heating modeling showed that the specimen ends heated more slowly (colder ends) than the center for the magnetic ferrite structure and more rapidly (hotter ends) for the non-magnetic austenite structure. Changing the specimen design to a thin wall tube allowed faster heating rates and modified the pattern of temperature variations within the test sample. The response of selected specimen geometries to induction heating in the dilatometer was characterized by modeling and tests using multiple thermocouples to verify the models. The results demonstrated that the use of properly designed tubular test specimens can aid in more accurately determining the A c1 transformation characteristics during heating at ultra-fast induction heating rates.

Historically, steels with carbon contents above about 0.45

Abstract Quenching and tempering (Q&T) allows a wide range of strength and toughness combinations to be produced in martensitic steels. Tempering is generally done to increase toughness, although embrittling mechanisms result in temperature ranges where strength and toughness may decrease simultaneously. Tempered martensite embrittlement (TME) represents one such mechanism, associated with the decomposition of retained austenite and precipitation of cementite during tempering, usually between 250 and 450 °C. The use of induction heating allows for time-temperature combinations, previously unobtainable by conventional methods, that have been shown to improve properties. The present work shows a beneficial effect of rapid tempering in alloy 1045, with an increase in energy absorption of about 50% when measured at room temperature via a three-point bending fracture test in the TME regime. Phase fraction measurements by Mössbauer spectroscopy showed that increased energy absorption was obtained despite essentially complete decomposition of retained austenite during tempering. Scanning electron microscopy (SEM) investigation of the carbide distribution showed refinement of the average carbide size of approximately 15% in the rapid tempered conditions. SEM characterization of the fracture surfaces of the rapid tempered three-point bend samples showed that, despite an increase in energy absorption in the TME regime, increased microscopic ductile fracture appearance was observed only at the highest test temperature.

It is well known that the element boron (B) improves the hardenability of low- to medium-carbon steels, reducing the need for expensive alloying elements. This study focuses on investigating the effect of quenching rate on B segregation at prior austenite grain boundaries (PAGBs) and Charpy impact energy in medium-carbon AISI 4130 steel with a tempered-martensitic microstructure. NanoSIMS data after quenching from austenite at different rates, including water quenching (WQ), He-gas quenching (HeQ), and Ar-gas program quenching (ArQ), were used to analyze B-doped 4130 steel specimens. The results revealed that the concentration of B along the PAGBs, and the extent of B segregation and boro-carbide precipitation increased with decreasing quenching rate. This phenomenon could be explained by the critical time required for non-equilibrium segregation of B during quenching. Charpy impact test data showed that the B-doped 41B30 specimens and slowly quenched specimens exhibited higher absorbed energy than the non-B-doped 4130 specimens and rapidly quenched specimens, respectively, particularly in the impact transition temperature region. This improvement in impact energy can be attributed to several factors. First, B doping enhances the resistance to brittle quasi-cleavage fracture by strengthening the grain boundaries through cohesive bonding. Second, B facilitates slip transfer across the grain boundaries, promoting plastic deformation and enhancing toughness. Finally, B doping may suppress the formation of transition carbides during tempering, further contributing to improved impact properties.

The effects of starting microstructure (hot-rolled vs lamellar pearlite (LP)-annealed) and microalloying (Nb vs Al) on the induction hardening response of 1045 steels were investigated. The hot-rolled microstructures of the steels consisted of pearlite and pro-eutectoid ferrite, while the LP anneal produced ferrite/pearlite banded microstructures. The 1045Nb steel had a lower initial ferrite fraction than the 1045Al steel. All samples after simulated induction hardening contained non-martensitic transformation products (NMTP), consisting of ferrite, bainite, pearlite, and retained austenite, in the martensitic matrix. The NMTP fraction generally depended on the prior ferrite fraction and size. For a microstructure exhibiting a higher fraction of ferrite and larger size of ferrite islands, complete austenitization during heating was slower. More homogeneous, post-induction, martensitic microstructures were obtained when Al was replaced with Nb, which is associated with decreased ferrite fraction in the pre-induction microstructure for the 1045Nb steel, rather than the direct alloying influence. Hardenability for a given induction thermal cycle was also closely related to the ferrite amount/size in the pre-induction microstructure. It is therefore concluded that prior processing to reduce ferrite fraction/size in the starting microstructure can positively contribute to induction hardenability by efficiently homogenizing the martensitic microstructure during rapid induction hardening.

Historically, steels with carbon contents above about 0.45% C that are quenched to attain hardness above about 53 HRC (560 HV) are prone to brittle intergranular fracture when stressed in uniaxial or cyclic tension. In this study, five laboratory-melted steels containing nominally higher 0.56% C, additions of Mn, Mo, Ni, or W, and no grain refining additions (Ti, Al, V, or Nb) were heat treated on a Gleeble 3500® simulator to emulate thermal heating and quenching cycles for both induction surface hardening and conventional through thickness heat treatment. Rapid heating at 50 °C/s and limiting the peak heating temperatures and times to 950 °C and 10 s produced a very fine austenite grain size (AGS) as small as 10 µm with a final hardness above 60HRC (700HV) and additions of Mo. Ni or W further refined the AGS to as small as 5 µm for the rapid heating. Fracture resistance measured by the peak breaking load (PBL) in notched bend tests increased by up to threefold for the short low-temperature heating cycles as compared to longer time (1000 s) higher-temperature (1050 °C) cycles. Fracture surfaces showed transgranular crack propagation for the short low-temperature cycles as compared to intergranular fracture for the longer higher-temperature cycles. The addition of 0.56 wt. pct W was especially effective for reducing AGS and PBL, and the addition of 2.12 wt. pct Ni increased the PBL for most of the heat treatment conditions.

Cold forging of carburizing steels has been utilized as a viable alternative to hot forging, but often increases the susceptibility to abnormal growth of prior austenite grains in localized areas associated with strain gradients induced during forging. The effects of microalloying with (Al + N) and (Nb + Mo) and strain (controlled here by reduction ratios during cold rolling) on abnormal grain growth (AGG) were investigated in medium-carbon low-alloyed modified AISI 4121 steels, heat treated to a final microstructure of martensite after simulated carburization at 1203 K. After cold deformation followed by the carburizing simulation, abnormally grown prior austenite grains were found in both (Al + N)-alloyed ‘AN’ and (Nb + Mo)-alloyed ‘NM’ steels. The number density of abnormally grown grains increased rapidly from a reduction ratio of 0 to 10 pct (true strain of 0.11) and then decreased to zero at a reduction ratio of 50 pct (true strain of 0.69). In the NM steel, the maximum prior austenite grain size and number density of abnormally large grains at a reduction ratio of 10 pct were larger than those of the AN steel, indicating a higher susceptibility to AGG in the NM steel. Complex (Nb,Mo)(C,N) carbonitrides present in the NM steel coarsened with time during long-time (328 minutes) high-temperature (1203 K) carburizing simulations. These results demonstrate that understanding AGG through microalloying and the amount of cold working play a crucial role in the wide application of cold-forging carburizing steels.

Higher carbon 1060 steel can be completely transformed to austenite at 850, a 100 °C lower temperature than that for conventionally used 1045 steel, allowing development of very small austenite grains during induction hardening. Torsion fatigue test specimens were machined from quenched and tempered martensite and as hot-rolled pearlitic bars of 1060 steel. Specimens from both initial conditions were induction hardened to a 3 mm case depth with peak surface temperatures of 1050 and 850 °C followed by tempering at 176 °C. Specimens were fatigue tested in torsion at R = 0.1 and peak stresses of 1222 to 1444 MPa, corresponding to 49 to 58

Carburizing and induction hardening are two commonly used surface heat treatments that increase fatigue life and surface wear resistance of steels without sacrificing toughness. It is hypothesized that induction hardening following carburizing could yield further increased torsional fatigue performance through reducing the magnitude of the tensile residual stresses at the carburizing case-core interface. If successful, manufacturers could see gains in part performance by combining both established approaches. A carburizing heat treatment with a case depth of 1.0 or 1.5 mm and an induction hardening heat treatment with a case depth of 0, 2.0, or 3.0 mm were applied to torsional fatigue specimens of 4121 steel modified with 0.84 wt pct Cr. The carburized samples without further induction processing, the 0 mm induction case depth, served as a baseline for comparison. The as-received microstructure of the alloy was a combination of polygonal ferrite and upper bainite with area fractions of approximately 27% and 73% respectively. The case microstructure of the heat-treated conditions was primarily tempered martensite and transitioned to a bainitic microstructure around the deepest overall case depth. Material property characterization consisted of radial cross-sectional hardness testing and torsional fatigue testing. The hardness profiles confirmed that the designed case depths were achieved for all conditions. Torsional fatigue testing was conducted using a Satec SF-1U Universal Fatigue Tester. Of the six tested conditions, the condition with the deepest case depths, i.e. carburized to 1.5 mm and induction hardened to 3.0 mm, was expected to have the greatest increase in fatigue performance. However, initial fatigue results potentially indicate the opposite effect as the non-induction hardened samples exhibited longer fatigue lives on average.

Carburizing is frequently utilized in the automotive industry in order to increase the surface hardness of a steel alloy while retaining toughness and ductility in the core. At elevated temperatures where some carburizing processes are performed, abnormal grain growth (AGG) can occur. During AGG, the microstructure undergoes bimodal grain growth with some grains growing exponentially faster than others. The growth of large austenite grains through AGG compromises the fatigue performance of carburized steels. AGG is further exacerbated by cold work introduced into the alloy prior to carburizing. Warm work is also sometimes utilized in part forming prior to carburizing. In this study, the effects of warm work on AGG were investigated. AISI 4121 and a modified AISI 4121 that contains Nb and Mo microalloying additions rather than Al for grain size control were warm worked in a range of 0-50% at a temperature of 900°C and then heated in a furnace for various lengths of time at a temperature of 930 °C to simulate a carburizing thermal history. The average prior austenite grain size (PAGS) tended to decrease as the degree of warm work increased, with the NbMo-modified alloy presenting a finer PAGS at all percentages of warm reduction and different lengths of time at the simulated carburization temperature. Specimens of the 50% warm reduced condition were also cold rolled at 5, 10, and 25% reductions, typical of cold sizing, prior to simulated carburization. The average PAGS of these CR samples was finer than their 0% CR counterparts, but the PAGS increased with CR in the modified alloy after 328 minutes of simulated carburization.

The objective of this work was conducted to investigate the influence of nickel (Ni) content and retained austenite on rolling-sliding contact fatigue (RSCF) life in carburized gear steel. In order to evaluate Ni and retained austenite effects, this study utilized carburized steel specimens of 4120 (0.13 wt pct Ni) and 4820 (3.38 wt pct Ni), which were subjected to RSCF testing. The specimens were gas carburized with a resulting case depth of approximately 1.3 mm, based on a hardness of 500 HV. The retained austenite was measured using x-ray diffraction at depths beneath the surface of 50, 250, 450, 650 μm. The 4120 specimens have a higher surface retained austenite content than the 4820. Specimens were surface ground to an average surface roughness of 0.2 μm to decrease the effect of as-carburized surface roughness on the fatigue life. The specimens underwent RSCF testing, with a surface contact stress of 2.5 GA and a slide to roll ratio of -20 pct, until a pit formed, as detected by an accelerometer. The pits that formed on the surface of the specimens were analysed with secondary electron microscopy, macrophotographs, and light optical microscopy. The pits that formed from the RSCF testing conditions were surface-initiated. The fatigue life of the 4820 specimens was higher than the fatigue life of the 4120 specimens, suggesting that the higher Ni level is beneficial to the fatigue life.

Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.

The addition of a small amount of boron (B) remarkably increases the hardenability of low-carbon low-alloy steels.1–3 This is because the segregation of B at prior austenite grain boundaries (PAGBs) may reduce the grain-boundary energy and promote the transformation from austenite to bainite or martensite while it prevents the transformation from austenite to ferrite.4,5 This B segregation is generally explained by the following equilibrium or non-equilibrium mechanism.6-8 Equilibrium segregation occurs by the movement of solute atoms from the grain-interior matrix to loosely-packed regions such as grain boundaries, thereby reducing the grain boundary energy. Non-equilibrium segregation occurs during the cooling from high temperatures by the diffusion of vacancy-solute complexes toward grain boundaries. Thus, during the cooling, the B segregation at PAGBs is mainly controlled by the non-equilibrium mechanism.4,7–19

Commercially, carbon steels are induction heated at heating rates on the order of 100 to 1,000 °C·s-1 for surface hardening. The high precision DIL 805L dilatometer employs induction heating and is often used to study transformation characteristics and prepare test specimens for metallurgical analysis. However, heating the commonly used 4 mm diameter by 10 mm long specimens at rates above 50 °C·s-1 results in non-linear heating rates during transformation to austenite and large transient temperature variations along the specimen length. These limitations in heating rate and variances from ideal uniform heating can lead to inaccurate characterization of the transformation behavior compared to commercial induction hardening practices. In this study it is shown that changing the specimen design to a thin wall tube allows faster heating rates up to 600 °C·s-1 and modifies the pattern of temperature variations within the test sample. The response of selected specimen geometries to induction heating in the dilatometer is characterized by modelling and tests using multiple thermocouples are used to verify the models. It is demonstrated that the use of properly designed tubular test specimens can aid in more accurately establishing transformation characteristics during commercial induction hardening.

A micro-alloyed 1045 steel was commercially rolled into 54 mm diameter bars by conventional hot rolling at 1000 °C and by lower temperature thermomechanical rolling at 800 °C. The lower rolling temperature refined the ferrite-pearlite microstructure and influenced the microstructural response to rapid heating at 200 °C·s-1, a rate that is commonly encountered during single shot induction heating for case hardening. Specimens of both materials were rapidly heated to increasing temperatures in a dilatometer to determine the Ac1 and Ac3 transformation temperatures. Microscopy was used to characterize the dissolution of ferrite and cementite. Continuous cooling transformation (CCT) diagrams were developed for rapid austenitizing temperatures 25 °C above the Ac3 determined by dilatometry. Dilatometry and microstructure evaluation along with hardness tests showed that thermomechanical rolling reduced the austenite grain size and lowered the heating temperature needed to dissolve the ferrite. With complete austenitization at 25 °C above the Ac3 there was little effect on the CCT behavior.

Carburization of steel involves a heat treatment of the steel surface using a source of carbon followed by quenching, which increases the hardness of the steel surface and subsurface. Gas carburizing, normally performed at temperatures in the range of 900°C to 950°C, is the most widely used carburizing method in industry.1,2 Higher-temperature vacuum carburizing increases the rate of carbon diffusion and makes it possible to reduce the processing times compared to gas carburizing.3 However, the high-temperature vacuum carburizing process can lead to excessive austenite grain coarsening when elevated temperatures are utilized to reduce cycle times.4. Modification of commercial carburizing alloys are being sought to achieve high temperature grain size stability and maintain or improve existing fatigue performance.

Precision cold-forging processes are used to produce near-netshape parts that may then be carburized. During carburization thermal cycles, abnormal grain growth (AGG) after cold forging is known to develop microstructures which limit fatigue strength. In the present study, a small 0.04 wt.% Nb addition was made to a low-alloyed AISI 4121 steel containing 0.3 wt.% Mo. Subcritically annealed specimens were cold rolled (to simulate cold forging) at selected reduction ratios up to 50%, heated according to a simulated gas carburizing cycle at 930 °C, and water quenched to produce a final martensitic microstructure. The number density of abnormally grown grains increased rapidly as the cold rolling reduction ratio increased from 0 to 10%. With a further increase in reduction ratio, the extent of AGG decreased and was absent in samples subjected to the maximum reduction ratio of 50%. The evolution of fine (Nb, Mo)(C,N) precipitates at various stages of processing was characterized by thermodynamic calculations and electron microscopy and compared to the occurrence of abnormal austenite grain growth. The significance of these results for controlling AGG and thus optimizing fatigue performance in commercially-produced cold-forged and carburized components is discussed.