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 A c1 and A c3 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 A c3 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 A c3 there was little effect on the CCT behavior.

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.

Rapid induction hardening of martensitic steel can attain the very high strength levels needed for light-weighting components subjected to high operating stresses. Specimens of martensitic 0.6% C steels were heat treated using a dilatometer to investigate the effects of heating rates of 5 to 500 °C/s to temperatures of 850 to 1050 °C on the transformation to austenite and subsequent transformation to martensite during quenching. Selected specimens were quenched after partial transformation to austenite to assess the initial cementite precipitate size formed in ferrite during heating. Other specimens were isothermally held at the austenitizing temperature to assess cementite dissolution rates. Higher heating rates increased the Ac1 and Ac3 temperatures, and lowered the Ms temperature. Alloy content and prior microstructure also influenced the transformation temperatures.

This paper investigates the factors that influence quenching rates and temperature distributions in steel during dilatometry testing. In a prior study, the authors assessed the performance of the cooling system in a widely used dilatometer. The goal of the current work is to develop a cooling strategy that provides more uniform and possibly faster cooling in the same system. Several alternate quench concepts are analyzed, the most promising of which uses water-cooled tubes to deliver high velocity gas through a series of jets axially aligned with the test sample. The proposed cooling apparatus and its effect on the induction heating process are assessed using CFD, electromagnetic, and thermal analyses.

Dilatometry test systems are commonly used for characterizing the transformation behavior in steels and induction heating is commonly the heating source. In these systems, the steel test article is assumed to have a uniform temperature throughout the sample. This is a good assumption for slow heating rates with small samples, however, for induction hardening cycles this may or may not be accurate. Using computer models, it is possible to predict the temperature dynamics of the sample, both radially and axially, during the thermal processing cycle (heating and cooling). O1 tool steel was utilized to characterize and model heating and cooling temperature gradients. Specimens instrumented with multiple thermocouples were induction heated and gas quenched. The test data and geometry were evaluated with 1- D and 2-D models to characterize transient temperature gradients. The goal of the modeling is to better characterize temperature corrections required when rapid heating and cooling processes are used to determine transformation behavior in induction hardenable steels.

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 premature brittle fracture when stressed in uniaxial or cyclic tension. In this study, five laboratory melted steels containing nominally 0.56% C 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 induction hardening. Limiting the peak heating temperatures and times produced very fine grained austenite with final hardness above 60HRC (700HV). Fracture resistance measured by the peak breaking load in notched bend tests increased by up to 3 fold for the short low temperature heating cycles as compared to longer higher temperature cycles. Fracture surfaces showed trans-granular crack propagation for the short low temperature cycles as compared to inter-granular propagation for the longer higher temperature cycles.

The austenite grain size (AGS) developed immediately before final cooling by quenching or other means is known to exert a substantial effect on the resistance to fracture in the final part. SAE 1045 steels containing Al, V, and V+Nb and a nonmicroalloyed steel were preconditioned by various thermomechanical treatments and then assessed for AGS by several methods defined in ASTM E112-13. Test results show that the microalloys present, the test method selected to evaluate the AGS, and the condition of the steel prior to conducting the grain size tests all have substantial effects on the measured AGS. The results demonstrate that for a meaningful AGS specification and test result, it is necessary to specify the precondition of the steel, the test method, and the test conditions.

Dilatometry and transmission electron microscopy were used to characterize the effects of V content, Si content, tempering temperature and starting microstructure on the hardness and microstructural evolution of a 0.4 wt pct carbon steel after a simulated nitriding thermal cycle. When tempered at 500 °C, significant amounts of V are left in solution leading to precipitation during the nitride thermal cycle increasing the hardness and dilation strain. Increases in Si content also lead to higher core hardness after nitriding, but Si does not significantly increase dilation strain during nitriding. Bainite starting microstructures produced less dilation strain during nitriding compared to martensite starting microstructures when tempered at 500 °C.

Microalloying of medium carbon bar steels is a common practice for a number of traditional components; however, use of vanadium microalloyed steels is expanding into applications beyond their original designed use as controlled cooled forged and hot rolled products and into heat treated components. As a result, there is uncertainty regarding the influence of vanadium on the properties of heat treated components, specifically the effect of rapid heat treating such as induction hardening. In the current study, the torsional fatigue behavior of hot rolled and scan induction hardened 1045 and 10V45 bars are examined and evaluated at effective case depths of 25, 32, and 44% of the radius. Torsional fatigue tests were conducted at a stress ratio of 0.1 and shear stress amplitudes of 550, 600, and 650 MPa. Cycles to failure are compared to an empirical model, which accounts for case depth as well as carbon content.