Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

On the microstructure and mechanical properties of high-speed steels

The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. The role of the primary carbides has been reported to be of no importance in strengthening the steels, due to their large size and large interparticle spacing. The present authors have studied the role of the primary carbides on the wear of high-speed steels and found them to be of no importance, and under certain conditions contributing to higher wear rates. It has been shown analytically and experimentally that in quenched and tempered high-speed steels, the precipitation of the secondary hardening carbide (cubic M₂C type) is the main reason for the improved strength and wear resistance. This shows that the secondary hardening phenomenon of high-speed steels is a direct result of the hardening caused by the precipitation of the cubic M_2C-type carbide. The present study has estimated that at peak hardness the volume fraction of secondary hardening carbides is approximately 20%. The measured strength of high-speed steels was found to be lower than the theoretically calculated strength due to non-homogeneous precipitation of the secondary hardening carbides. Areas which were observed to be free from secondary hardening carbides are real and are not artefacts. It has been shown that the strength of high-speed steel in the region of peak hardness depends primarily on the precipitation of the secondary hardening carbide and secondarily on martensitic strengthening.     

Effect of microstructure on tensile fracture of quenched and lightly tempered high carbon and low chromium steel containing undissolved spheroidal carbides

High carbon and low alloy chromium steels have been studied to determine the effect of the microstructure on tensile fracture of quenched and lightly tempered low alloy steel containing undissolved spheroidal carbides. The steels with a volume fraction of 8 and 13 vol % and containing particle sizes from 0.32 to 1.14m were investigated. In the case of steel containing 8 vol % undissolved carbides, many twinned plates were observed in the matrix martensite and microtwinning was observed in the carbide/matrix interfaces. The steel failed in a macroscopically brittle manner and the true fracture stress of the steel was independent of the carbide particle size, while the data exhibited a large scatter. In the case of steel containing 13 vol % of undissolved carbides, the matrix martensite consisted predominantly of lath martensite and a well-defined forest of dislocations was observed around the carbides. Failure of the steel occurred in the relatively early stage of plastic deformation and the true fracture stress of the steel increased with decreasing carbide particle size.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness.Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M₇C₃ types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M₂C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M₆C developing at the expense of the fine M₇C₃ carbides previously formed.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

Chromium carbide laser-beam surface-alloying treatment on stainless steel

In order to improve the resistance to wear, oxidation and corrosion of a stainless steel die, chromium carbide surface-alloying treatment was carried out on a 12 % Cr stainless steel using a CO₂ laser. Cr₃C₂ powder slurry was coated on the stainless steel and then a 3 kW CO₂ laser beam was used to irradiate the specimen. The thickness of surface-alloyed layer was about 0.3 mm and the chromium concentration was about 40 % throughout the alloyed-region. Large amounts of Cr₃C₂ and Cr₇C₃ were also distributed in this alloyed layer. From the results of the isothermal oxidation test at 960 °C for 100 h, it was found that the surface-alloying treatment improved the oxidation resistance by about 100 times due to the distribution of chromium carbides and the increase in the chromium concentration. The results of the cyclic oxidation test revealed that the oxidation layer was very stable on the chromium carbide surface-alloyed region, while it scaled off very easily from the substrate region due to porous oxidation products. The microhardness was about 1100 H_v due to the dispersion and precipitation of chromium carbides in addition to the martensitic structure in the surface-alloyed region. The microhardness did not decrease much, despite heating at 960 °C for 100 h. The chromium carbide surface-alloying treatment improved the wear-resistance greatly, and the results of the wear-resistance test were very consistent with those of the microhardness test.     

The formation of chromium carbide during chromizing

The carbides formed during the chromizing of various types of carbon and chromium steels are considered in terms of the ternary phase diagram. A correlation is found between the carbide or carbides formed and a diffusion couple model. In low chromium, high carbon steels, an intermediate layer is formed which seems to be a (Fe,Cr)₃C cementite phase. The carbide which is formed on low carbon constructional steels depends on the detailed carbon and chromium profiles. Data found in the literature support the present interpretation.     

High-temperature properties and microstructural stability of hot-work tool steels

Indexable insert tools for machining operations are in service exposed to high temperatures and cyclic mechanical loads. Secondary hardening steels such as hot-work steels are commonly used for tools subjected to thermal exposure. However, these steels, highly alloyed with strong carbide forming elements as Cr, V and Mo, are generally difficult to machine and machining represents a large fraction of the production cost of a tool. Thus, the present study concerns the development of a new steel with improved machinability and meeting the requirements for high-temperature properties.Softening resistance of the THG2000 and QRO90 tool steels, commonly used in hot-work applications, and a newly developed tool steel MCG2006 with lower alloying content of carbide forming elements, was investigated by tempering and isothermal fatigue testing. Mechanisms of high-temperature softening of the tested tool steels were discussed with respect to their microstructure and high-temperature mechanical properties. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis.No difference in softening behaviour was found among the QRO90 and MCG2006 regarding hot hardness measurements. The THG2000 indicated some stabilization of the hardness between 450 and 550 °C and a considerable hardness decrease at higher temperatures.The short-time cyclic softening in isothermal fatigue was controlled by dislocation rearrangement and annihilation. The alloying composition of the steels presently tested had no influence on the dislocation density decrease.The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance. The QRO90 with greater molybdenum and lower chromium contents than in the THG2000 show the best resistance to softening among the tested grades at all temperatures. The MCG2006, leaner alloyed with the carbide forming elements and alloyed with 4 wt% nickel, has better temper resistance than THG2000 at higher temperatures and longer tempering times.     

Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.     

Microstructure evolution in bulk and surface states of chromium rich nickel based cast alloys reinforced by hafnium carbides after exposure to high temperature air

Abstract

Three alloys based on nickel, with a high level of chromium (25 wt-%) and containing varied quantities of carbon, 0·25 and 0·50 wt-%, and hafnium, 3·7 and 5·6 wt-%, fabricated by casting, were subjected to a 46 h long exposure at 1200°C in dry industrial air. The aim of the work was to investigate the thermal stability of their carbide interdendritic network and to discover their general behaviour in high temperature oxidation. The volume fraction of the hafnium carbides decreased slightly during high temperature exposure but their fragmentation was rather limited. In contrast, chromium carbides appeared in the two alloys, which initially contained exclusively HfC, and this may result in a decrease in their high temperature properties. The evolution of the carbides appeared to be responsible for a moderate lowering of room temperature hardness. The behaviour of the three alloys during high temperature oxidation was very good, despite the unusually high content of hafnium. All were chromia-forming, although oxidation of Hf led to HfO₂ islands in the external scale and in the subsurface region. In summary, the behaviour of these three alloys showed that the HfC containing Ni–25Cr family is potentially interesting for applications at very high temperatures.     

Effect of selective oxidation of chromium on creep strength of Alloy 617

Abstract

In high temperature alloys, the selective oxidation of chromium to form a chromia scale leads to subsurface depletion of chromium in the alloy which in turn leads to the dissolution of chromium carbide in the depleted zone. The effect of this carbide depleted subsurface zone on the creep properties of Inconel Alloy 617 (Ni–22Cr–12Co–9Mo–1Al–0·08C; wt-%) has been determined. The specimens were subjected to heat treatments before creep testing to simulate long term service exposure of a thin walled heat exchanger tube operating at high temperatures. It was found, surprisingly, that in creep tests carried out at 900°C, specimens having extensive chromium depleted and carbide free subsurface zones exhibit higher creep strength than specimens thermally aged for the same durations, but having no chromium depleted zone. As chromium was removed from the matrix owing to selective oxidation, the carbon, released as the carbides in the chromium depleted zone dissolved, migrated to the centre of the specimen, producing enhanced carbide precipitation. This led to an increase in the creep strength of the specimen core, which offset the loss in creep strength of the subsurface zone. The expected detrimental effect of chromium depletion was therefore not observed.

MST/1487     

Metallographic changes in the sintering of grade T high speed steels in an industrial atmosphere and vacuum

The metallographic changes taking place in three T grade steels, T42, T15 and T1 in the range of optimum sintering temperature to oversintering in vacuum and in an industrial atmosphere — base nitrogen — have been investigated using SEM and EDS techniques. M₆C and MC primary carbides have been observed for vacuum sintering and MX carbonitrides instead of MC carbides were found in the specimens sintered in the atmosphere. Small amounts of an eutectic carbide rich in Cr and Fe have been observed at the optimum sintering temperature. By oversintering in vacuum different type of eutectic carbides can be observed: MC, needle shape, M₆C, but only M₆C type eutectic carbide have been observed by oversintering the samples in the industrial atmosphere.     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264     

Wear and oxidation behaviour of high-chromium white cast irons

High-chromium white cast irons can attain very hard microstructures composed of intergranular chromium carbides dispersed in a tempered martensitic matrix. Two heat-treated high-chromium white cast irons with different carbon contents were studied in this research work using compression at 500 °C, and wear and oxidation tests at 500 and 600 °C. The hot compression behaviour of these products is excellent, with high strengths and significant ductility. They also exhibited good wear and oxidation properties. Fracture micromechanisms are discussed in relation to the chemical composition and microstructure of the two alloys.     

Metallographic observations during the sintering of BM2 type of high speed steels

Metallographic changes during the sintering of BM2 type of high speed steels have been investigated by scanning electron microscopy, in the range of optimum sintering to oversintering. The primary carbides observed are M₆C and a small quantity of MC; in the oversintered structure an additional carbide with eutectic morphology was seen. It is a chromium and molybdenum rich phase in BM2+ 0 to 4% cobalt (0.9 to 1.2% carbon) alloy, whereas in BM2+ 8% cobalt (0.9% carbon) the eutectic phase is MC. Under certain conditions M₃C was also detected in the post-sintered alloy.     

Effect of carbon content on microstructure and corrosion behavior of hypereutectic Fe–Cr–C claddings

The aim of this study was to examine the influence of carbon content on the microstructures and corrosion characteristics. The results showed that the hypereutectic microstructure comprised primary (Cr,Fe)₇C₃ carbides and the eutectic colonies [γ-Fe + (Cr,Fe)₇C₃]. The amounts of primary (Cr,Fe)₇C₃ carbides increased from 33.81 to 86.14% when carbon content increased from 3.73 to 4.85 wt%. The corrosion resistance of the hypereutectic alloy with 4.85 wt% C was about 20 times higher than that with 3.73 wt% C. The galvanic corrosion occurred in all claddings due to difference of corrosion potential between primary carbide and austenite. The dense distribution of primary carbides could retard the austenitic matrix from selective corrosion. The austenite dissolved the Fe²⁺ ions and formed a Cr₂O₃ film under 3.5% NaCl aqueous solution.