Showing papers in "Journal of Heat Treating in 1981"

A study of reactions occurring during the carbonitriding process

Abstract: The reactions occurring during the carbonitriding process have been studied theoretically and experimentally. A model for the prediction of the amount of residual ammonia in the furnace from the process parameters, temperature, inlet gas composition and flow-rate is presented. It takes account of ammonia decomposition and formation of the hydrogen cyanide. The ammonia addition and decomposition also cause a dilution of the carburizing atmosphere resulting in a shift in the gas equilibria and the carbon potential. A method to take this into account is presented. The examination of the formation of hydrogen cyanide and its behaviour in the atmosphere leads to the conclusion that hydrogen cyanide is of major importance as a nitriding component. All carburizing and nit riding reactions are listed together with equilibrium and rate constants.

Influence of sub-zero and shot-peening treatments on impact and fatigue fracture properties of case-hardened steels

Abstract: High-cycle fatigue endurance limits and impact fracture strengths were determined for carburized SAE 4028 steel with and without a shot-peening treatment and for carbonitrided SAE EX55 steel with and without a sub-zero treatment at -85 °C (-120 °F). The shot-peening and the sub-zero treatments did not influence the impact fracture strengths of the steels; the impact fracture strengths of carburized SAE 4028 and carbonitrided EX55 steels were 2250 MPa (325 ksi) and 3220 MPa (468 ksi), respectively. The treatments had marked influence on the fatigue endurance limits of the steels, however. The fatigue endurance limit of carburized SAE 4028 was 730 MPa (105 ksi) without the shot-peening treatment and 1035 MPa (150 ksi) in the peened condition. The carbonitrided EX55 steel had endurance limits of 965 MPa (140 ksi) without the sub-zero treatment and only 415 MPa (60 ksi) after the treatment. The differences in fatigue endurance limits were explained through changes in residual stress patterns in the hardened cases. Shot-peening induced a compressive residual stress in the surface of carburized SAE 4028 steel, which was as high as 650 MPa (94 ksi). The residual stress in the case of EX55 without the sub-zero treatment was compressive for both the martensite and austenite phases, whereas the stress existing in the austenite after the sub-zero treatment was highly tensile. Differences in residual stresses among individual phases within the microstructure were used to explain observed changes in fracture mode.

Prediction of hardenability from isothermal transformation diagrams

Abstract: An analytical method for evaluation of quantitative hardenability of eutectoid steels from the corresponding TTT diagram was considered. The isothermal transformation kinetics of pearlite and cooling curves were combined by the additivity rule, and an equation expressing continuous cooling transformation of pearlite has been derived. Based on this equation, simple algorithms for the calculation of the widely used measure of hardenability, such as the critical cooling rates, the Jominy distance, and the ideal critical diameter from the corresponding TTT diagram were presented. It was shown that the derived equations give the above measures of hardenability with good accuracy. Finally, the relationships between J0 and D1 and among D1, D0, and H were reexamined taking cooling transformation into account. It was found that the formerly obtained relationships based on the half temperature time required correction. New relationships based on the cooling transformation are presented.

A Study of Reactions Occurring during the Carbonitriding Process Part II

Abstract: The presented theoretical treatment is a continuation of a former paper (J. Heat Treating, 1981, vol. 2, no. 1). A simple expression based on the geometric exclusion model for the interaction between carbon and nitrogen in austenite is derived. A method for the calculation of the carbon and nitrogen potentials of the carbonitriding atmosphere is presented based on the rate equations for the relevant reactions. Calculated values are compared with experimental ones. The central part of the paper is the calculation of carbon and nitrogen profiles in carbonitrided steels. The carbon and nitrogen concentration dependent diffusion coefficients for carbon and nitrogen are included as well as the interaction between alloying elements and carbon and nitrogen in steel. Calculated and experimentally derived concentration profiles are compared for various types of heat treatment cycles like two-step boost and diffusion treatments.

X-ray determination of retained austenite

Abstract: The variation of retained austenite content in heat treated steels is studied here by an X-ray diffraction technique. It is found that the retained austenite content in steel samples studied increases with the austenitizing temperature and decreases slowly with the tempering temperature up to 177 °C. Above this temperature, it decreases very rapidly until at the tempering temperature of 260 °C it becomes completely zero. The effects of surface preparation on the results of X-ray diffraction measurements are also demonstrated.

High cycle fatigue resistance of AISI E9310 carburized steel with two different levels of surface retained austenite and surface residual stress

Abstract: The high cycle fatigue resistance of carburized AISI E9310 steel has been investigated for two different levels of surface retained austenite and surface residual stress. Fatigue specimens were carburized to a case depth of 1.9 mm with a surface carbon content of 1.0 wt pct carbon. This treatment gave surface retained austenite and surface residual stress levels of 56 vol pct and -595 MPa (-86 ksi), respectively. Half of these specimens were then given a refrigeration treatment which lowered the surface retained austenite level to 31 vol pct and raised the surface residual stress level to —760 MPa (-110 ksi). High cycle fatigue tests showed that the specimens with the higher surface retained austenite level had the best high cycle fatigue resistance. X-ray diffraction studies showed that both treatments of steel exhibited decreasing retained austenite and residual stress levels with cycling. Light microscopy and electron fractography showed that fracture apparently initiated in the carburized case, although the exact initiation site could not be determined. A nalysis of the results suggests that the poorer high cycle fatigue resistance for the steel with the low retained austenite results from a degradation of the fatigue resistance of the case material.

Heat treating processes with nitrogen and methanol-based atmospheres

Abstract: The increasing costs of energy and the variations in hydrocarbon composition have increased the interest in the heat treatment of steels with nitrogen based atmospheres. Recently, new processes have been developed using nitrogen and small additions of fluids. The nitrogen-methanol process enables a pure carrier gas to be used. The purity and constant composition of the atmosphere confer simplicity and reliability to the process. The quality and consistent high purity of nitrogenmethanol atmospheres guarantee uniform case depths of carburized products without sooting and with the desired carbon potential. The control and monitoring of the carburization are easy and reliable to run with the recording of carbon dioxide or oxygen content. The main advantages of this type of atmosphere are independence from natural gas composition and supply, a reliability and flexibility of the process for all types of heat treatment, a real control of carbon potential, and flow rate reductions applied to the type of furnace. This type of nitrogen methanol atmosphere is applied either to neutral hardening, carburizing or carbonitriding or to carbon restoration and sintering of carbon steels. This paper discusses a number of recent industrial developments in the field of thermochemical treatments of automotive parts.

A practical approach to carburizing process determination

Abstract: This article describes an approach to determining carburizing processes by knowing and controlling the basic parameters of surface carbon concentration, time, temperature, carbon potential of the furnace atmosphere, and end resulting effective case depth. Provided in this paper are the mathematical and graphical means for determining processes that will produce a given case depth.

Annealing of Cold-Rolled Rephosphorized Steels Containing Si and Cb

Abstract: The effects of continuous and batch annealing on the mechanical properties and strengthening mechanisms of rephosphorized steels containing Si and Cb were studied. Cold rolling in combination with continuous annealing provided a higher yield strength than was attained in the hot-rolled material because of the resultant finer ferrite grain size. On the other hand, batch annealing following cold reduction generally resulted in a lower yield strength because of grain growth and, for the Cb-alloyed steels, precipitate coarsening as well. For the steels alloyed with Si, in the case of batch annealing, increasing Si from 0 to 0.5 pct resulted in increasing yield stength due to both grain refinement and solid solution strengthening, whereas in the case of continuous annealing, the Si addition resulted solely in solid solution strengthening. For the Cb-steels, the increase in yield strength with increasing Cb content was due to increased precipitation strengthening in the case of continuous annealing and due to both increased precipitation strengthening and grain growth retardation in case of batch annealing.

The effects of phosphorus and carbon on the hardenability of 41XX type steels

Abstract: The effects of phosphorus and carbon on the hardenability of 41XX steels were examined by Jominy end-quench testing. Carbon was evaluated by selection of 4130, 4140, and 4150 steels, and each alloy contained nominally 0.002 or 0.02 wt pct P. Hardenability was significantly enhanced by phosphorus (an increase in DI, of about 35 pct when P was increased from 0.002 to 0.02 pct) in the 4150 steel only. Isothermal transformation experiments and the Jominy tests showed that the improvement in hardenability was due to the retardation of bainite formation by phosphorus in the 4150 steel. The improved hardenability is attributed to critical concentrations of segregated carbon and phosphorus at austenitic grain boundaries, apparently the same phenomenon that renders medium carbon steels susceptible to tempered martensite embrittlement.

Effect of composition on distance to first bainite in carburized steels

Abstract: The effects of Mn (05 to 11 pct), Mo (00 to 05 pct), Ni (00 to 15 pct) and Cr (00 to 10 pct) on retarding the formation of bainite in carburized steels were studied using 81 different alloy compositions Jominy bars for all the alloys were carburized at 925 °C (1700 °F), cooled to 845 °C (1550 °F) and end quenched Flats were ground on the bars to a depth corresponding to 09 pct case carbon content, hardness profiles were determined and the flats were metallographically examined to determine the distance from the quenched end of the bar at which bainite first forms, the so-called distance to first bainite (DFB) Multiple regression analysis was applied to the data to yield an empirical relation for DFB vs alloy content An analysis of variance was also applied to the DFB data and revealed that those elements which have a statistically significant effect on retarding bainite formation are molybdenum (strongest effect), chromium (strong effect) and manganese (moderate effect) The effect of nickel is of borderline statistical significance and the weakest of all the elements

Recrystallization of 70 Pct Cu-30 Pct Zn brass by laser irradiation and furnace heating

Abstract: A laser heating technique together with conventional heating has been used to determine the recrystallization kinetics of 40 pct cold worked 70:30 brass in the temperature range of 250 to 670 °C and time range of 106 to 10-2 s. The coherent radiation of a pulsed ruby laser heated thin specimens (0.002 in.) at a rate of 106 °C/s. He-H2O vapor quenching produced quench rates of 105 °C/s. Quantitative metallography and microhardness measurements were used to follow recrystallization kinetics. The isothermal analyses of kinetic data indicate that the volume fraction recrystallized, f, can be expressed as f = 1-exp (-atn) where the value of n was found to be 1.70 and essentially independent of temperature. This value of n indicated a preferred nucleation and two dimensional growth of recrystallized regions. The analysis of kinetic data on grain boundary motion showed that the grain boundary velocity G, is inversely proportional to time of annealing, t, with the relationship G = (2.29 × 10-2)/t and its temperature dependence can be expressed as G = 1.58 × 108 exp (-35,247/RT). The microhardness data indicated that recovery processes occur to a greater extent during recrystallization at low temperatures than during high temperature-short time annealing.

The role of a vacuum furnace in heat treating coated tool steels

Abstract: A comparative study of the retained austenite, grain size, hardness, and carbide precipitates was made on M-2 and M-4 high speed steels coated by chemical vapor deposition (CVD) and hardened by the salt bath method and vacuum gas quench method. Critical process parameters were identified. Hard coating of HSS and tool steel tooling with titanium carbide and titanium nitride by the CVD method is an accepted rapidly growing surface hardening process.

Thermochemistry of exothermic gas atmospheres

Abstract: Annealing processes often use exothermic gas as a furnace atmosphere to protect against oxidation. A mathematical model of such a process was used to determine the effects of generator temperature, heat exchanger temperature, generator air-methane ratio and furnace temperature on the equilibrium composition of the furnace atmosphere. For temperatures above 700 °C, the only gas phase reaction which need be considered is the water gas shift reaction. At lower temperatures it is possible to form methane from CO and H2 and, below about 600 °C, CO can decompose resulting in loss of carbon from the atmosphere. Both methane formation and carbon loss tend to make the furnace atmosphere more oxidizing. Because of slow reaction rates, neither the formation of methane nor loss of carbon is likely to approach equilibrium, even if the water gas shift reaction reaches equilibrium. The implications of these factors for furnace atmosphere control are discussed.

Evaluation of gas carburizing in a new fluidized-bed furnace

Abstract: A newly designed, internally fired, air: propane fluidized bed furnace is described and used to carburize a low carbon steel. The increased carburizing rates, compared with the conventional sealed-quench furnace, are achieved without the problem of sooting even at carbon potentials of 2.3 pct C. Since the measured activation energy was found to be of the same order as that for carbon diffusion in austenite, it was postulated that the increased carburizing rates were due to the increased heat transfer rates, increased carbon potential, and the kinetically more favorable surface conditions existent in the fluidized bed.