The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron

The fracture toughness of a high chromium, reduced carbon white cast iron was measured using the K_Ic fracture toughness test. The toughness was found to increase with increasing heat treatment temperature for the temperature range of 1273–1423 K. Increases in the fracture toughness were due to crack deflection into the dendritic phase. Cracking in the dendrites was promoted by the presence of secondary carbides which formed during the high temperature heat treatment employed. The characteristic distance for brittle fracture as calculated by the Ritchie–Knott–Rice model correlated well with the centre to centre mean free path of the secondary carbides on the fracture plane.     

Kinetics of precipitation in Al-1 wt% Mn alloy

Precipitation kinetics in high purity Al-10 wt% Mn alloy has been investigated during the early stages of isothermal annealing between 823 and 698 K by resistivity measurements. Aged specimens were also examined by transmission electron microscopy. The precipitation kinetics can be represented by an Avrami type equation with the time exponentn=2/3 during the early stages of annealing. This can be attributed to the nucleation and growth of fiat needle-like precipitates on dislocations. The precipitates were Al₆ Mn at 773 K and the metastable G₂ -phase at 773 K. After prolonged annealing times at 848 K, isometric plates of Al₆Mn were also observed and the value ofn was found to be 1.0. However, at 698 K after longer annealing times, isometric needles of Al₁₂Mn, G-phase were observed along with flat needles of G₂ precipitate. At 698 K, the value ofn tends to decrease with time from 0.67 to 0.59.     

Formation of spheroidal carbide in vanadium white cast iron by rare earth modification

Abstract

In this paper spheroidal VC carbide was produced by rare earth (RE) + N complex modification of white cast iron and the mechanism of modification was investigated. Primary crystallisation of VC in the melt, required for its independent growth into spheroids, was prompted by high V content of the alloy and heterogeneous nucleation. Nuclei in spheroidal VC were found to contain the compositions of the modifier. The formation of effective duplex nuclei from RE + N complex modifier was thermodynamically favourable. Cubic VC tended to dendritic growth in the alloy melt owing to S segregation in the melt close to cubic corners. Thus, purification of the melt by RE preventing dendritic growth favoured the spherical growth of VC. The effects of impurity and modifier elements on the spheroidisation of VC were proved further by the alteration of carbide morphology after adding these elements either separately or simultaneously to pure Fe–C–Valloy. High cooling rates did not favour the spherical growth of VC.

MST/1151     

Microstructure and mechanical properties of high boron white cast iron with about 4 wt% chromium

The microstructure and mechanical properties of high boron white cast irons with about 4 wt% chromium before and after treating with rare earth magnesium alloy were studied in this article. The experimental results indicate that the cast irons comprise a dendritic matrix and interdendritic eutectic borides M₂B and M′_0.9Cr_1.1B_0.9 that distributed in the form of continuous network in as-cast condition. The matrix is made up of fine pearlite in the alloys with and without modification, but the grain size of the matrix is decreased greatly after modification. After water quenching at 1,303 K and tempering at 473 K, the matrix of the alloy mostly changes to lath-type martensite. For the alloy without modification the boride morphology remains almost unchanged after heat treatment. And a secondary precipitation of M₂₃(C,B)₆ compound appears in the central region of dentritic matrix grains. The morphology of the eutectic borides is changed to the form of isolated blocks after heat treatment and there is only little intragranular M₂₃(B,C)₆ particles in the matrix are found in the alloy modified with rare earth magnesium alloy. The modification by rare earth magnesium alloy can refine the primary austenite and the eutectic borides. Combined with a high austenitizing temperature the modification can improve the morphology of the borides which results in the improvement of toughness and tensile strength.     

Spinodal decomposition in Co-3wt% Ti-1wt% Fe and Co-3 wt% Ti-2 wt% Fe alloys

The transformation on ageing Co-3 wt% Ti-1 wt% Fe and Co-3 wt% Ti-2 wt% Fe alloys has been followed by transmission electron microscopy and diffraction to establish that the solid solutions undergo spinodal decomposition at 823 and 973 K. The microstructural evolution has been correlated with the observed variations in hardness and yield strength. The coarsening of the modulations on ageing is seen to follow ³- ₀ ³ =Kt kinetic law. After long periods of ageing a discontinuous coarsening process sets in.     

The precipitation of niobium carbide at grain boundaries in an austenitic stainless steel

Electron microscopy has been used to show that the precipitation of niobium carbide at the grain boundaries of an austenitic stainless steel can occur in a spatially non-random fashion. Preferential nucleation and growth of carbides occurs on grain-boundary defect structures. Where present, these defect structures include extrinsic grain-boundary dislocations and topographical discontinuities. An example of the precipitation of niobium carbide on an intrinsic dislocation array is also shown.     

Strain aging and carbide precipitation in aluminium-killed steels

Abstract

The effect of 0·003–0·050 wt-%C additions on strain aging and carbide precipitation in batch-annealed aluminium–killed steels has been investigated. The low–carbon steels exhibited unusual strain–aging characteristics: steels containing 0·020–0·040 wt–%C strain aged at 160 but not at 50°C, whereas those containing 0.007–0·020 wt–%C aged at both temperatures. This behaviour is due to the presence of fine carbide particles, which form as a result of the supersaturation of carbon in the matrix during cooling and which raise the carbon solubility by the Thomson-Freundlich effect. Expressions are derived which give the carbon supersaturation as a function of temperature during cooling, and of the precipitation temperature of each class of carbide. Carbide precipitation alternates between the nucleation of new particles and the growth of existing ones, depending upon the particle spacing and the cooling rate. Strain aging in the 0·05 wt–%C steel is negligible up to 225°C, but then rises to a peak at 325°C, which has been attributed to effects of partition in the dislocation matrix.

MST/141     

TEM study on precipitation and transformation of secondary carbides in 16Cr–1Mo–1Cu white iron subjected to subcritical treatment

High chromium white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The subcritical heat treatment can destabilize the austenite by precipitating chromium-rich secondary carbides and other special carbides. In the as-cast condition the eutectic carbides are (Fe,Cr)₇C₃ and (Fe_4.3Cr_2.5Mo_0.1)C₃. The initial secondary carbide precipitated is (Fe,Cr)₂₃C₆ after heat-treating at 853 K for 10 h. There are MoC, Fe₂MoC and -carbide precipitating, and (Fe,Cr)₂₃C₆ transforms to M₃C after 16 h at 853 K. The -carbide and (Fe,Cr)₂₃C₆ accomplish transformation to M₃C and the matrix changes from martensitic to pearlitic after 22 h at 853 K. Thereby, in the subcritical heat treatment process, the initial secondary carbide precipitated is (Fe,Cr)₂₃C₆, followed by -carbide, MoC and Fe₂MoC. In addition, there are two in situ transformations from (Fe,Cr)₂₃C₆ and -carbide to M₃C carbides.     

Microscopic modelling of simultaneous two-phase precipitation: application to carbide precipitation in low-carbon steels

A thermodynamically-based precipitation model, employing the classical nucleation and growth theories, has been adapted to deal with simultaneous precipitation of metastable and stable phases. This model gives an estimation of the precipitation kinetics (time evolution of radius and density of precipitates for both phases, as well as the evolution of solute fraction) in a wide range of temperature. Results were successfully compared with an experimental isothermal precipitation diagram (Time–Temperature-Transformation, TTT) from the literature for the precipitation of ε carbide and Fe₃C in low-carbon steels.     

热轧态Inconel690合金中碳化物的溶解和析出

研究了热轧态690合金中碳化物的溶解和析出行为及其结构,结果表明:在热轧态合金中存在的碳化物多数沿晶界长条状分布,少量呈颗粒状分布于晶内,类型为M_(23)C_6.热轧态合金的晶界和晶内碳化物的完全固溶温度分别为1050℃、1080℃,在低固溶温度下未完全溶解的残余晶界碳化物直接导致后续TT处理晶界不再析出碳化物;将合金完全固溶处理后,在后续TT处理的晶界上会重新析出细小、半连续的碳化物.     

The high-temperature creep behaviour of an Al-1 wt% Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ～ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.     

Hot workability of an Al-Mg alloy AA5182 with 1 wt% Cu

A comparative study of the hot workability of two aluminium alloys, alloy AA5182 used for automotive applications and a variant modified with 1 wt% copper, has been carried out. Hot torsion tests were performed on both alloys subjected to two different heat treatments: a low temperature preheat to 450 °C and a high temperature preheat at 540 °C. The results from the torsion experiments are interpreted in terms of microstructural features. Both treatments produce the same strength, but the high temperature preheat leads to better ductility. This improvement is related to the homogenization of solute elements in the matrix; and, concerning AA5182 + Cu, also to the dissolution of a non-equilibrium Al-Mg-Cu ternary eutectic present in the as-cast microstructure. The precipitation of (Fe, Mn)Al₆ precipitates in the matrix of both alloys is induced by the high temperature heat treatment. Comparison of the results obtained by hot torsion shows that at low deformation rates AA5182 + Cu has better ductility than the classical alloy, but its ductility is lower at strain rates above 0.6–0.8 s^–1. The null ductility transition temperature is lower compared with that in the classical alloy, restricting the range of hot working temperatures. Inside this range the strength of both alloys is approximately the same, although the strain rate sensitivity coefficient is increased by copper additions. The experimental strength values follow the classical sinus-hyperbolic constitutive equation for hot working.     

X-ray investigation of carbide precipitation in 2·25Cr–1Mo steel for predicting remaining life of boiler components after extended service in fossil fuel fired power stations

Abstract

X-ray diffraction (XRD) studies were carried out on 2·25Cr–1Mo steel boiler tubes from a thermal power station with an objective of developing a faster method of health assessment of this steel at elevated temperatures on a routine basis. Carbide phase transformations in virgin and service exposed boiler tubes (up to ～100 000 h) were determined for plate specimens using high power X-rays. Iron and chromium rich carbides were detected by XRD at various stages of transformation towards final equilibrium. Compositional variations of equilibrium precipitate M₂₃ C₆ with service life or overheating in the specimens studied were monitored by a shift in the lattice parameter. The XRD results were validated by optical and electron microscopy, hardness measurements, and particle size measurements.     

Isolation,identification and quantification by X-ray diffraction of carbide phases in 2 1/4Cr-1 Mo steel

The addition of controlled amounts of phosphorus to 2 1/4Cr-1 Mo steel, heat treated under two different conditions, has produced microstructures containing a wide spectrum of types and amounts of individual carbide phases. X-ray diffraction analysis of carbide precipitates has been undertaken following quantitative extraction by anodic dissolution of the matrix. By combining this with selective chemical dissolution of certain carbide types, calibrations of relative weight fractions against the intensities of suitable X-ray diffraction peaks may be obtained without the need for the addition of a standard calibration powder. This has allowed the determination of the absolute amounts, within a given steel sample, of the carbide types M₆C, M₇C₃, M₂₃C₆ and, most importantly, the creep-strengthening M₂X phase.     

Kinetics of niobium carbide precipitation in a low carbon austenitic steel

An examination has been made of the kinetics of niobium carbide precipitation in a 18-10-1 austenitic stainless steel in the temperature range 650 to 750° C. Electrical resistance-time plots, thin film electron microscopy and hardness measurements have been employed to follow the ageing sequence. In these alloys the carbides precipitate on undissociated dislocations and in association with stacking faults; these processes are diffusion controlled and have an activation energy of ～318 kJ mol^?1. Prior to the reaction beginning a clear incubation period existed, e.g. ～30 h at 650° C and 20 min at 750° C. During the first 10% transformation the carbide nucleation rate increases and the associated faults nucleate and grow rapidly. The carbide nucleation rate appears to peak around this level and then falls away gradually to zero around 70% transformation. At this latter stage fault growth ceases, and transformation continues by a carbide growth process. The age-hardening peak occurs much beyond the end of the reaction by which time precipitate coarsening is in evidence. The precise effect mechanical deformation has upon stacking fault formation depends to a major extent on the niobium supersaturation in the quenched alloy.