Coarsening resistance of M2C carbides in secondary hardening steels: Part III. Comparison of theory and experiment

A model for the coarsening resistance of multicomponent carbides was used to study the effect of Mo and Cr on the coarsening kinetics of M₂C carbides in commercial AF1410 and experimental alloy steels. Experimental studies of coarsening behavior of the carbides in these steels have been made by using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The measured coarsening rate constant agrees with model predictions within a factor of 2 to 3. The coarsening kinetics of M₂C carbides in these alloys is found to be controlled by the volume diffusion of alloying element M. A Cr-Mo alloy steel with the predicted optimum composition showed the slowest coarsening kinetics and highest hardness at long tempering times.     

Novel hafnium containing steels for power generation

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M₂₃C₆ carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M₂N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M₂₃C₆ grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part I. Theoretical model for multicomponent coarsening kinetics

The development of very high-strength levels in many alloy steels is achieved by a secondary hardening reaction. In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Coarsening resistance of the M₂C precipitates depends on the alloy content of these elements, and there should be an addition to the alloy of these carbide-forming elements which optimizes the M₂C coarsening resistance. Current Lifshitz-Slyozov-Wagner (LSW) theory^[2,3] cannot properly be used to describe, the coarsening behavior of multicomponent carbides, which involves concentrations and diffusivities of two or more solutes and nonspherical carbide morphologies. A model is introduced for the coarsening resistance of multicomponent carbides. This model treats the coarsening of shape-preserving particle and is applicable to rodlike particles.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part II. Alloy design aided by a thermochemical database

In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Thermodynamic stability and coarsening resistance of these carbides depend on the alloy content of these elements. A model for the M₂C coarsening kinetics in multicomponent alloys has been used to identify the optimum alloying addition for maximum coarsening resistance and as a basis for selection of four experimental alloy steels. Necessary information pertaining to the equilibrium in these steels was obtained using the Thermo-Calc software and database developed at the Royal Institute of Technology, Stockholm, Sweden.     

Modeling Creep Strength of Welded 9 to 12 Pct Cr Steels

The influence of weld-simulated heat treatments of 9 to 12 pct steels is evaluated by a fundamental model for creep. The heat-affected microstructure is predicted by considering particle coarsening, particle dissolution, and subgrain coarsening. Particle coarsening is predicted for a multicomponent system, showing significant M₂₃C₆ coarsening in the bcc matrix. Dissolution simulations of MX and M₂₃C₆ are performed by considering a size distribution of particles, indicating that the smallest particles can be dissolved already at relatively low welding temperatures. Recovery in dislocation networks will take place due to the coarser particles. Creep rate modeling is performed based on the heat-affected microstructure, showing strength reduction of weld-simulated material by 12 pct at 1123 K (850 °C) and 30 pct at 1173 K (900 °C). The main cause of this degradation is believed to be the loss of the smallest carbonitrides.     

High-Performance dispersion-strengthened Cu-8 Cr-4 Nb alloy

A new high-temperature-strength, high-conductivity Cu-Cr-Nb alloy with a Cr:Nb ratio of 2:1 was developed to achieve improved performance and durability. The Cu-8 Cr-4 Nb alloy studied has demonstrated remarkable thermal and microstructural stability after long exposures at temperatures up to 0.98 T_m. This stability was mainly attributed to the slow coarsening kinetics of the Cr₂Nb precipitates present in the alloy. At all temperatures, the microstructure consists of a bimodal and sometimes trimodal distribution of strengthening Cr₂Nb precipitates, depending on precipitation condition, i.e., from liquid or solid solution, and cooling rates. These precipitates remain in the same size range, i.e., large precipitates of approximately 1 μm and small precipitates less than 300 nm, and effectively pin the grain boundaries, thus retaining a fine grain size of 2.7 μm after 100 hours at 1323 K. This grain-boundary pinning and sluggish coarsening of Cr₂Nb particles explain the retention of good mechanical properties after prolonged holding at very high temperatures, e.g., twothirds of the original yield strength after aging for 100 hours at 1273 K. The main sources of strengthening are the Hall-Petch and Orowan mechanisms due mostly to small particles. The coarsening kinetics of the large precipitates are most likely governed by grain-boundary diffusion and, to a lesser extent, volume diffusion mechanisms.     

Effect of plastic deformation on the coarsening of

Particle coarsening has been studied in a rapidly solidified Al-8.8 wt pct Fe-3.7 wt pct Ce alloy subjected to isothermal annealing for various times at 425 °C. The effect of static and dynamic loading on the particle coarsening rates at the same temperature also has been examined. The dispersed particles in all specimens of the present study are the equilibrium Al¹³Fe⁴ and Al¹⁰Fe²Ce phases. They are incoherent with the matrix and constitute 23 pct of the total volume. The coarsening rate in isothermally annealed specimens is orders of magnitude greater than predicted by the modified Lifshitz-Slyozov-Wagner theory for volume diffusion controlled coarsening but can be explained using Kirchner’s model for coarsening by diffusion along grain boundaries. In the case of intragranular particles, coarsening by diffusion along dislocation cores also is likely to be significant. Creep loading is seen to cause a significant enhancement of the coarsening rate. Fatigue testing with a hold period at the maximum tensile stress also accelerates coarsening whereas continuous cycling appears initially to retard the increase in the average particle size. Dislocations connecting dispersed phase particles are observed more frequently in crept specimens and specimens fatigued with a hold period than in specimens fatigued with no hold period. The effects of plastic deformation on particle coarsening rates are discussed in terms of excess vacancy generation, short circuiting along dislocations, and fine precipitation during fatigue.     

Effect of Co on Creep Behavior of a P911?Steel

The microstructure and creep behavior of a 3 pct Co modified P911 steel and standard P911 steel were examined. It was shown that the nanoscale M₂₃C₆ carbides and MX carbonitrides in the 3 pct Co modified P911 steel are not susceptible to significant coarsening under creep conditions. Also, coarsening simulations of M₂₃C₆ particles were performed for both steels. The rates of lath and particle coarsening in the P911 + 3 pct Co steel are remarkably lower than those in the P911. Increased stability of a tempered martensite lath structure in the 3 pct Co modified P911 steel provides enhanced creep resistance at an exceptionally high temperature of 923 K (650 °C).     

The Computational Design of W and Co-Containing Creep-Resistant Steels with Barely Coarsening Laves Phase and M23C6 as the Strengthening Precipitates

Generally, Laves phase and M₂₃C₆ are regarded as undesirable phases in creep-resistant steels due to their very high-coarsening rates and the resulting depletion of beneficial alloying elements from the matrix. In this study, a computational alloy design approach is presented to develop martensitic steels strengthened by Laves phase and/or M₂₃C₆, for which the coarsening rates are tailored such that they are at least one order of magnitude lower than those in existing alloys. Their volume fractions are optimized by tuning the chemical composition in parallel. The composition domain covering 10 alloying elements at realistic levels is searched by a genetic algorithm to explore the full potential of simultaneous maximization of the volume fraction and minimization of the precipitates coarsening rate. The calculations show that Co and W can drastically reduce the coarsening rate of Laves and M₂₃C₆ and yield high-volume fractions of precipitates. Mo on the other hand was shown to have a minimal effect on coarsening. The strengthening effects of Laves phase and M₂₃C₆ in the newly designed alloys are compared to existing counterparts, showing substantially higher precipitation-strengthening contributions especially after a long service time. New alloys were designed in which both Laves phase and M₂₃C₆ precipitates act as strengthening precipitates. Successfully combining MX and M₂₃C₆ was found to be impossible.     

Microstructural aspects of the causes of type IV cracking in Cr-Mo steel weld joints

Fusion welded joints of Cr-Mo steels fail prematurely under creep condition at the heat affected zone (HAZ) close to the base metal, termed as type IV cracking. Optical metallography and hardness testing across the joint establish that the type IV cracking occurs in the soft intercritical HAZ. Based on detailed microstructural studies carried out to understand the evolution of the microstructure and its role in determining the tendency for type IV cracking, the factors that lead to deterioration of creep strength in intercritical HAZ in weld joint of Cr-Mo steels are:

1. fine grained structure
2. coarse M₂₃C₆ carbides at grain and sub-grain boundaries
3. dissolution of M₂X and MX types of intragranular precipitates.

In the case of low Cr steels, the dissolution of intragranular Mo₂C is an important factor among others in determining the tendency to type IV cracking in the weld joint. On the other hand, in higher Cr alloys, M₂₃C₆, which plays a dominant role in determining substructure strengthening by stabilizing the substructures, is found to be the main cause of type IV cracking in the weld joint. The dissolution of finer M₂₃C₆ and the accompanying coarsening of the large particles leads to the modification of lath-like substructure having high dislocation density into fine polygonal ferrite having low dislocation density, which in turn reduces the creep strength profoundly. The preferential Z-phase formation accompanied with the dissolution of intragranular (Nb,V)(C,N) in the intercritical HAZ is also considered as a factor for the type IV cracking on longer creep exposure. The paper would highlight and discuss in detail some of our results on these lines.     

Diffusion-controlled growth and

The growth and coarsening kinetics of MnS precipitation were investigated during the hot deformation of electrical steels. The size distributions of the MnS particles in the three tested steels were followed at 800 °C, 900 °C, and 1000 °C, and the relevant precipitation start(P _s) and finish(P _f) times were also determined. The results show that the growth of these precip- itates obeys a parabolic law and is controlled by the diffusion of Mn atoms. Since the Mn concentration at the particle/matrix interface X¹ _Mn is far below the equilibrium solubility X^e _Mn, the growth rate increases with increasing overall Mn concentration. Analysis of the experimental data demonstrates that the effective diffusivity of Mn during particle growth is increasingly modified by pipe diffusion as the temperature is decreased. The experimental results regarding coarsening indicate that the kinetics during this stage are limited mainly by bulk diffusion at the lowest temperature. However, both bulk and grain boundary diffusion processes become rate controlling at the two higher temperatures. The latter process appears to become more and more important as the temperature is raised, a trend which can be attributed to the location of an increasing fraction of the MnS precipitates at the grain boundaries.     

Microstructural changes during overtempering of high-speed steels

To elucidate the mechanisms determining the creep resistance of high-speed steels during tool service, overtempering at 600°C has been investigated for two alloys modeling the matrix compositions of AISI M2 and T1. Composition changes and coarsening of the secondary hardening precipitates were studied by transmission electron microscopy and field-ion microscopy with atom probe analysis. Strengthening in the peak-hardened state is due to coherent precipitates of types M₂C and MC. During overtempering, M₂C coarsens too rapidly to be of importance for the sustained strength of the material. The MC precipitates, on the other hand, are fairly stable. Some coarsening does occur, but the MC population is replenished by a second wave of precipitation which makes use of the roughly 50 pct of carbide-forming elements, carbon, and nitrogen, which remained in solid solution after tempering to the peak-hardened state. This precipitation reaction continues for times of the order of the tool life.     

Kinetics of MnS precipitate coarsening in 3pct Si-Fe sheet

The growth kinetics of MnS in a 3 pct Si-Fe matrix containing excess manganese were characterized at temperatures of 925°, 975°, and 1025°C by an electron microscopy extraction replica technique. The average radius of coarsened particles at timet, •r _t, exhibited Greenwood’s predicted dependence for diffusion-controlled coarsening; namely, r3t= r3 0+kt where r_-o is the critical radius for growth andk is the rate constant. Growth rates compared favorably to those calculated from diffusion-controlled coarsening theories assuming that growth was controlled by the diffusion of sulfur. The experimentally determined activation for coarsening, ~67 kcal, was attributed to the sum of the activation for the dissolution of MnS, ~24 kcal, and for the diffusion of sulfur, ~49 kcal.     

Optimisation of long-term creep strength of martensitic steels

Besides the reduction of greenhouse gases the increase of thermal efficiency is one of the major goals in modern material development for process and power plants. Increasing the steam inlet temperatures and pressures is at present the favoured method to increase the thermal efficiency. For the realization of 700°C power plants, new creep resistant ferritic-martensitic 9–12 wt. % Cr steels are required to be applied in the 650°C temperature range. An important task for the optimization of long term creep properties is the characterization of the changes in the microstructure during creep exposure. A sufficient long term creep strength is based on a small initial size and slow coarsening of the M₂₃C₆ precipitates as well as the dynamic precipitation of small V(C, N) particles along with the absence of Z-phase. The paper describes the R&D activities of the MPA University of Stuttgart in the frame work of national and international research projects aimed at the development and long term characterisation of optimised martensitic steels with higher long term creep strength.     

On methane generation and decarburization in low-alloy Cr-Mo steels during hydrogen attack

Low-carbon, low-alloy Cr-Mo steels may fail by hydrogen attack when they are exposed to high hydrogen pressures at elevated temperatures. During this process, the dissolved hydrogen reacts with the carbides of the steel to form methane in grain boundary cavities. The methane pressure inside these cavities depends on the microstructure of the used steel, which consists of a ferritic matrix and alloy carbides such as M₇C₃, M₂₃C₆, M₆C, and M₂C. The different phases in the multicomponent system Fe-Cr-Mo-V-C are modeled with the sublattice model. Their Gibbs energies are then used to calculate the equilibrium methane pressure as a function of the microstructure. Driven by the methane pressure, the cavities grow due to grain boundary diffusion and dislocation creep, which is described by analytical relations. This leads to progressive development of damage inside the material but, at the same time, to a decrease of the carbon content in the steel. This reduction depends on, among other factors, the methane pressure and the damage state. As the carbon content also affects the creep parameters, this process of decarburization may accelerate the cavity growth. Model calculations are used to obtain insight into the influence of this decarburization process on damage evolution and the final lifetime.     

Response of Phase Transformation Inducing Heat Treatments on Microstructure and Mechanical Properties of Reduced Activation Ferritic-Martensitic Steels of Varying Tungsten Contents

Microstructure and mechanical properties of 9Cr-W-0.06Ta Reduced Activation Ferritic-Martensitic (RAFM) steels having various tungsten contents ranging from 1 to 2 wt pct have been investigated on subjecting the steels to isothermal heat treatments for 5 minutes at temperatures ranging from 973 K to 1473 K (700 °C to 1200 °C) (below Ac₁ to above Ac₃) followed by oil quenching and tempering at 1033 K (760 °C) for 60 minutes. The steels possessed tempered martensite structure at all the heat-treated conditions. Prior-austenitic grain size of the steels was found to decrease on heating in the intercritical temperature range (between Ac₁ and Ac₃) and at temperatures just above the Ac₃ followed by increase at higher heating temperatures. All the steels suffered significant reduction in hardness, tensile, and creep strength on heating in the intercritical temperature range, and the reduction was less for steel having higher tungsten content. Strength of the steels increased on heating above Ac₃ and was higher for higher tungsten content. Transmission Electron Microscopy (TEM) investigations of the steels revealed coarsening of martensitic substructure and precipitates on heating in the intercritical temperature range, and the coarsening was relatively less for higher tungsten content steel, resulting in less reduction in tensile and creep strength on intercritical heating. Tensile and creep strengths of the steels at different microstructural conditions have been rationalized based on the estimated inter-barrier spacing to dislocation motion. The study revealed the uniqueness of inter-barrier spacing to dislocation motion in determining the strength of tempered martensitic steels subjected to different heat treatments.     

Effect of fine precipitation and subsequent coarsening of Fe<Subscript>2</Subscript>W laves phase on the creep deformation behavior of tempered martensitic 9Cr-W steels

The effect of fine precipitation and subsequent coarsening of Fe₂W Laves phase on the creep deformation behavior was investigated for simple 9Cr-W steels containing 0, 1, 2, and 4 wt pct W. After tempering, the specimens were subjected to creep tests at 823, 873, and 923 K for up to 15,000 hours. The precipitation of Fe₂W Laves phase takes place during creep at boundaries from the supersaturated solid solution of the high-W steels, the 9Cr-2W and 9Cr-4W steels, but not in the low-W steels, the 9Cr-0W and 9Cr-1W steels. The fine precipitation of Fe₂W Laves phase decreases the creep rate in the primary or transient creep region, while the subsequent large coarsening of Fe₂W Laves phase reduces the precipitation strengthening and promotes the acceleration of creep rate in the tertiary or acceleration creep region after reaching a minimum creep rate. The change in shape of creep rate curves with stress and temperature is explained by taking fine precipitation and subsequent coarsening of Fe₂W Laves phase into account.     

How TEM Projection Artifacts Distort Microstructure Measurements: A Case Study in a 9 pct Cr-Mo-V Steel

Morphological data obtained from two-dimensional (2D) and three-dimensional (3D) transmission electron microscopy (TEM) observations were compared to assess the effects of TEM projection errors for submicron-size precipitates. The microstructure consisted of M₂₃C₆ carbides in a 9 pct Cr-Mo-V heat resistant steel before and after exposure to creep conditions. Measurements obtained from about 800 carbides demonstrate that particle size and spacing estimates made from 2D observations overestimate the more accurate values obtained from 3D reconstructions. The 3D analysis also revealed the M₂₃C₆ precipitates lengthen anisotropically along lath boundary planes, suggesting that coarsening during the early stage of creep in this alloy system is governed by grain boundary diffusion.