Internal oxidation during endothermic gas carburizing was studied in one plain carbon and three alloy steels. The oxide phases formed were analyzed using combined scanning Auger microscopy and thin window energy dispersive x-ray spectroscopy. The work verifies that the morphology of the internal oxides consists of two zones. In the outer zone near the carburized surface (zone 1) particles of Cr-Mn oxides form both within grains and on grain boundaries, and in the deeper zone (zone 2) a Si rich oxide is present exclusively on grain boundaries. Auger depth profiling of the Si rich oxide layer on the grain boundaries in zone 2 shows that it consists primarily of a mixture of SiO2 and Fe2SiO4. Point analysis on polished and sputter cleaned flat sections and depth profiles on fracture surfaces both indicate that the Si rich oxide layers along the grain boundaries are around 120 nm thick.
This paper describes some of the advanced surface modification processes now being applied to steels. The processes use vacuum chambers and electric and magnetic fields to create plasmas, and high energy beams, and are applied to produce enhanced surfaces of a wide variety of depths and properties. Four classes of treatments are considered: thermal, thermochemical, coating or plating, and ion implantation. Examples of the most widely applied treatments which are described include: plasma nitriding of medium carbon steel, physical vapor deposition of very hard titanium nitride coatings on tool steels, and localized laser hardening of steels and cast irons. An emerging trend is the combination of two advanced techniques in order to provide even higher levels of surface performance.
An embrittled 18 Ni maraging steel rolled ring was examined and samples cut from the ring were subjected to a wide range of heat treatments including high temperature solutioning and thermal cycling. The effects of these treatments on toughness were evaluated by measuring impact energy and plain strain fracture toughness. The microstructural analyses were carried out using extensive optical and scanning electron microscopy, and scanning electron fractography. It has been established that the ring was embrittled due to the combined effects of deformed structure and grain boundary precipitation of TiC or Ti (CN). Heat treatment parameters have been devised to improve the fracture toughness and grain size of the materials affected by these two types of embrittlement. It has been suggested that toughness and grain size can be improved by (a) annealing at 1223 K followed by water quenching in the case where deterioration in toughness is marginal and is caused by nonrecrystallized grains or deformation texture, and (b) solutioning at 1473 K followed by water quenching, and thermal cycling twice between room temperature and 1198 K with a holding time of 30 min at peak temperature in the case where the loss in toughness is considerably large due to excessive grain boundary precipitation of second phase particles.
Using a miniature gold plated copper disk as target, quenching experiments were performed with water sprays to correlate heat fluxq′’ to surface-to-fluid temperature difference AT, and the local values for the spray hydrodynamic parameters of volumetric fluxQ′’, mean drop velocityU m and Sauter mean drop diameterd 32 over a wide range of operating conditions (Q′’ = 0.58 x 10−3−9.96 x 10−3 m3 sec−1/m2,U m = 10.1-29.9 m/sec,d 32 = 0.137-1.350 mm), and surface temperatures up to 520° C. Drop diameter was found to have a weak effect on heat transfer in film boiling for all the conditions tested. Two distinct spray cooling regimes were identified, allowing the classification of sprays with respect to volumetric flux, low flux sprays forQ″< 3.5 x 10−3 m3 sec−1/m2, and high flux sprays forQ″ > 3.5 x 10−3 m3 sec−1/m2. WhileQ″ had a significant influence on film boiling in both regimes, drop velocity was important only for the high flux sprays. A spray quenching test bed was also constructed to simulate, under controlled laboratory conditions, spray quenching of alloys in an industrial environment. The test bed was used to generate temperature-time records for a rectangular aluminum plate during spray quenching. Using the software package ANSYS, the measured temperature response was successfully simulated by utilizing the newly developed boiling correlations in defining boundary conditions for the quenched surface after accounting for spatial variations in the hydrodynamic parameters within the spray field. The effectiveness of this numerical technique for the tested configuration is proof that it may be possible to predict the temperature-time history for quenched parts with complicated shapes provided the spatial distributions of the hydrodynamic parameters are well mapped or predetermined.
In order to economize on raw materials for producing controlled atmospheres and to avoid the air pollution caused by exhausted atmospheres, the feasibility of carburizing steel by using recirculated water-gas atmospheres was investigated. Water-gas was generated in a closed system and an open system respectively, and steel samples were heated under the water-gas atmospheres. Based on the heating results of steel, the relationship between the generation conditions of water-gas and its carburizing behavior was examined. In addition, the carburizing results as well as the charcoal consumption for both systems are compared with each other.
The various reactions occurring in nitrocarburizing atmospheres are described and discussed based on thermodynamic principles such as equilibria and reaction kinetics. Utilizing a model for nitriding and nitrocarburizing systems is shown to be beneficial in designing a furnace atmosphere with specific properties, that is, with different nitriding and carburizing properties. Furthermore the influence of different parameters, such as the nitrogen and carbon activities, on the constitution of the compound layer is demonstrated. The analysis of the nitrocarburizing atmosphere and the control of the process are also described and discussed.
In this paper the authors report the results of a series of experiments on two types of concrete steels subjected to rapid heat treatment (RHT). The results show that mechanical properties can be improved dramatically by the RHT.
In the laser surface hardening process, the thermal stress is induced by the temperature gradient and the martensitic phase transformation. The martensitic phase transformation is a dominant factor in formation of the residual stress. For the accurate prediction of internal stresses during laser surface hardening treatment, therefore, a particular effort is needed to model the progress in the martensitic phase transformation. In this study, the effect of the transformation plasticity was considered by using an additional strain in the numerical analysis of laser surface hardening treatments. The simulation results showed that the deviation between the residual stresses when considering the transformation plasticity and when not considering it on the hardened surface is large, so that it cannot be considered a negligible factor. In the nonhardened region, however, the shapes of the isostress lines were very similar to each other. Experimental results obtained by irradiating the test specimen with a 2.4 kW CW-CO2 laser showed a better agreement with the residual stress predicted by considering the transformation plasticity than by neglecting it.
The effect of various parameters, namely amount of prestrain, time of aging, and temperature of aging on the response, that is the increase in the yield strength, of a dual-phase steel was studied by applying statistical design of experiments. A regression equation was developed between the response and the parameters. The equation was corrected by Student “t” test and was found to be adequate as checked by Fisher “F” test and random experiment. Within the range of variables studied, the amount of prestrain, the time of aging, and the temperature of aging had negative, insignificant, and very significantly positive influence, respectively. An attempt was made to achieve a maximum for the response surface. The maximum was found to result from a combination of the parameters such as 0.5% tensile prestrain, aging time of 12 min, and aging temperature of 320°C.
The effect of cold rolling and subsequent annealing on the tensile mechanical properties, grain structure, and ear formation has been determined for a 3104 Al alloy. The starting material was directionally cast, hot rolled plate (440–493° C) (840–920° F) which was not completely recrystallized. This material was heated at 14° C/hr (25° F/hr) to an annealing temperature of 332° F (630° F), then held for 2 hr. This slow heating simulated that of batch annealing of rolled coils. Material in this annealed condition was cold rolled various amounts to 96% reduction in thickness, then annealed at 149–332° C (300–630° F) for up to 32 hr.
A hot etching procedure used to reveal the prior austenitic grain size of several steels was evaluated. Etching experiments, using molten glass, were conducted on a HSLA precipitation hardened steel, two alloy steels, and a control rolled steel. For comparison purposes, the austenite grain size of the HSLA precipitation hardenable steel was determined using a hot-stage microscope. The prior austenitic grain sizes obtained for these steels using the molten glass etch is presented.
Specimens from a series of carburizing steels with constant compositions except for variations in sulfur content, 0.006, 0.015, and 0.029 wt%, were machined from bar stock into cantilever type fatigue specimens. The specimens were carburized at 927° C (1700° F), oil quenched, tempered at 150° C (300° F), and tested in bending fatigue. Endurance limits of 1260 MPa (183 ksi), 1200 MPa (174 ksi), and 1070 MPa (155 ksi) were determined for the 0.006 wt% S, 0.015 wt% S, and 0.029 wt% S specimens, respectively. The major effect of sulfur was to increase the scatter in fatigue performance. All fracture was initiated by intergranular cracking, and elongated sulfides close to or intersecting the fatigue specimen surfaces apparently lowered the applied stresses required to initiate incipient intergranular cracks.
A reduction in the magnitude of out-of-round (OOR) and improved flatness of heat treated but unfinished tapered roller bearing components was achieved by carburizing parts at temperatures less than 927°C (1700°F) in a controlled, integral-quench process. Distortion results from this study are compared to conventionally processed components. Reasons for reduced distortion levels achieved are discussed in terms of temperature uniformity, atmosphere control, quenchant temperature, quenchant flow and postquench processing steps, product fixturing, and product orientation in the furnace load.
A low C steel containing 1.5% Mn, 1.2% Si, and 0.1% Mo was inter-critically annealed at 760 and 820° C, and either quenched in oil to room temperature, or isothermally held at 400° C and then oil quenched to room temperature. The specimens were subjected to tensile testing at temperatures between − 80° C and 120° C, and the microstructures were evaluated by light and scanning electron microscopy and x-ray diffraction. The austenite formed during intercritical annealing transformed to martensite with negligible retained austenite as a result of the direct oil quenching, while it transformed to bainite with significant amounts of retained austenite, up to 7.5%, during isothermal transformation. The retained austenite improved ductility at strength levels between 700 and 800 MPa by strain induced transformation of austenite to martensite. The benefit of the retained austenite peaked at testing temperatures between −30 and 70° C.
This study aims at developing a universal approach to predicting the temperature—time characteristics for three-dimensional aluminum parts during quenching with water sprays. The validity of correlations recently developed by one of the authors, which relate the local heat transfer rate to the spray hydrodynamic parameters, is examined for nonuniform sprays. A mathematical relation is presented for characterizing the spatial distributions of the hydrodynamic parameters. By combining the local heat transfer correlations with the spatial distribution equations of these parameters, it is shown how a spatial distribution of the surface heat transfer rate can be derived. This distribution provides the vital boundary conditions needed to numerically simulate the three-dimensional heat diffusion occurring in a spray cooled part and the resultant temperature—time cooling curve at every point in the part. The validity of this approach is demonstrated by comparing numerical predictions with experimental measurements on a three-dimensional rectangular aluminum block subject to a nonuniform spray cooling boundary.
A tempering process of alloy Ti-5Al-lMo-2Nb-3Zr has been investigated by using transmission electron micrograph (TEM), x-ray, electric resistance, and positron annihilation techniques in this study. After quenching from 950°C, the alloy consists of a phase and hcp martensite α′. Thin rectangular plates of twins appear in acicular martensites along a\(\left\{ {1 0 \bar 1 1} \right\}\) crystallographic plane. In tempering below 450°C, no obvious change has been observed with TEM. However, positron annihilation technique has detected some preparation process for martensite precipitation. Between 450 and 600°C, all measures show ß phase separates from martensite. Twin boundary and martensite boundary are the preferred precipitating sites with different nucleation mechanisms. The process proceeds most actively around 600°C. At temperatures higher than 600°C, TEM shows discontinuous β particles distributing along martensite boundaries and inside of martensite plates with Burger’s orientation. Heating up to 800°C, an interface phase appears during air cooling. A monolithic layer of the interface phase has been identified as fee lattice with the orientation\(\left( {1 1 0} \right)_\beta //\left( {0 0 1} \right)_m ,\left[ {1 \bar 1 \bar 1} \right]_\beta //\left[ {1 1 0} \right]_m \). The striated layer of the interface phase has been identified as\(\left\{ {1 0 \bar 1 1} \right\}\left\langle {1 0 \bar 1 2} \right\rangle \) twin to a phase.
A modification of the numerical method that takes into account continuous correction of the transfer coefficient D was suggested. It is expected that the suggested modification will yield better calculational values in cases where the transfer coefficient D is very dependent on the transfer of quantityC. The modification is suitable for computer application and is recommended for all analogous transfer phenomena (heat, mass, impulse, …) in metal heat treating.
An overview of manufacturing processes and heat treatments and their use in near-net shape part processing applications is given. Recent advances in process/power interfacing and their beneficial effects are covered as well.
Heat treating experiments, where the austenitizing temperature was varied, were conducted on HSLA-80 steel in order to determine if the impact properties, particularly at −17.8° C and −84° C, could be increased to those of HSLA-100. The impact properties were found to decrease as the austenitizing temperature increased and were dependant upon plate thickness. Selected heat treatments conducted on the 19 mm, 32 mm, and 51 mm thick plates produced acceptable impact results. The trend observed in the 19 and 32 mm thick plates was not as pronounced in the 51 mm thick plate due to its inability to attain a sufficient cooling rate. Impact tests were also conducted on specimens taken from the 19 mm thick plate and given a double austenitizing treatment. The effects of E-copper and niobium carbide precipitates on the impact strength are discussed.
