Hans-Olof Andrén

σ-PHASE PRECIPITATION IN STABILIZED AUSTENITIC STAINLESS STEELS

Experimental observations of sigma-phase precipitation in two stabilized austenitic stainless steers, AISI 321 and AISI 347, aged up to 80,000 h at temperatures between 500 and 800 degrees C, are c ...

Gradient zones in WC-Ti(C, N)-Co-based cemented carbides : Experimental study and computer simulations

The formation of surface zones with a composition gradient during sintering of WC-Ti(C,N)-Co cemented carbides has been studied experimentally and by computer simulations. The microstructure has been investigated with SEM and EPMA. The simulations are based on a solution of the multicomponent diffusion equations, coupled with thermodynamic calculations using thermodynamic descriptions of the individual phases. The results from the simulations are in good agreement with the experimental results, indicating that diffusion and the thermodynamic properties are the two major factors that control the gradient structure formation.

Effect of carbon content on the microstructure and mechanical properties of (Ti, W, Ta, Mo)(C, N)–(Co, Ni) cermets

Five model alloys with different carbon content were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with energy dispersive X-ray analysis (EDX). Vickers hardness (HV) and transverse rupture strength (TRS) were measured, and continuous turning tests were performed. The aim of this work was to relate the variation in carbon content to the resulting microstructures and to the mechanical behaviour of these model alloys. An increased carbon content resulted in decreased concentrations of tungsten and titanium in the binder phase, a lower volume fraction of undissolved Ti(C, N) cores, and a higher volume fraction of heavy (Ti, W, Ta)(C, N) cores. The volume fractions and compositions of the other phases were not much affected by the carbon content. In addition, a coarser carbonitride grain structure was observed in the material with a high carbon content. Hardness was found to decrease and TRS to increase with carbon content, but no obvious trend was found for the wear resistance as a function of carbon content. Several microstructural features interact and influence the final properties. The heavy (Ti, W, Ta)(C, N) cores formed during sintering and a highly solution hardened binder phase seem to be favourable for a high wear resistance.

Microstructure of model cermets with high Mo or W content

Abstract The microstructure of (mol%) TiC–18TiN–24Ni–(10–29)WC and TiC–18TiN–24Ni–(5–14)Mo2C has been investigated using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and analytical electron microscopy (AEM). When the WC content in the raw materials was increased the W content in the outer rim of (Ti, W)(C, N) grains increased until it had a composition similar to that of the inner rim. If the WC content was high undissolved WC was present after sintering. When the Mo2C content in the raw materials was increased, the volume fraction of inner rim increased and the Mo content in both inner and outer rim increased. Thermodynamical calculations on the Ti–W–C–N system suggest that the inner rim is formed during solid state sintering when there is an open porosity and thus a low nitrogen activity. The composition of the outer rim can be explained by the equilibrium at the sintering temperature if the volume fraction of undissolved Ti(C, N) cores is subtracted. Calculations on the Ti–Mo–C–N system show that (Ti, Mo)(C, N) decomposes into two phases with different Mo content and that the Ti(C, N) cores might be regarded as a stable phase.

Microstructures of cemented carbides

Abstract An overview is given of the detailed microstructure of cemented carbides of types WC–Co and WC–MC–Co and of titanium carbo-nitride based cemented carbides (cermets). The manufacturing process is first descibed, and the changes in composition that occur during manufacturing are discussed. The microstructure and its formation during sintering is then described for the three types of cemented carbides. In particular, the following are treated: Grain boundary segregration in the carbide skeleton; Control of solid solution hardening in the binder; Segregration in the binder; Formation of a core-rim structure in hard phase grains; Formation of an inner rim during solid state sintering; Slow diffusion of metal atoms in hard phase; Retention of phases far from equilibrium; Approach to paraequilibrium during sintering.

Microstructure development during sintering and heat-treatment of cemented carbides and cermets

Abstract WC based cemented carbides and Ti(C, N) based cermets are used in cutting tool applications. They are produced by liquid phase sintering, but much of the microstructure is formed already during solid state sintering, before the eutectic temperature has been reached. Changes in the microstructure during the sintering process have been followed with microscopy and microanalysis, in particular SEM and TEM including energy filtered transmission electron microscopy (EFTEM). It was found that part of the hard phases are dissolved in the solid binder, transported by diffusion and re-precipitated onto undissolved hard grains with a composition given by the equilibrium conditions (“inner rim”). For vacuum sintering of nitrogen containing materials, this means a low nitrogen activity due to the open porosity at this stage. After the liquid has formed, further dissolution and re-precipitation occur, but now the porosity is closed so the “outer rim” is formed with the nitrogen activity of the material. The solid solution of the binder (often by tungsten) is determined primarily by the carbon activity in the liquid phase. During cooling after sintering, tungsten and other metals dissolved in the binder re-precipitate onto hard grains so that depleted zones around these are formed, whereas dissolved carbon and nitrogen have time to leave the binder almost completely. Sintering or post-sintering heat-treatment of nitrogen containing materials in an atmosphere with a lower or higher nitrogen activity than in the material results in the formation of surface zones with a composition different from the bulk (“gradient sintering”). In the former case, a tough zone enriched in binder and depleted of cubic carbides is created, which is beneficial if the material is going to be coated with a wear resistant layer. In the latter case a hard surface zone rich in cubic carbo-nitrides and depleted of WC and binder may be obtained, improving the wear resistance of the material.

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti(2)AlC grains in different materials as well as on large M(23)C(6) precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the (13)C isotope, and calculating the concentration of (12)C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti(2)AlC an additional difficulty arises from the overlap between C(2)(+), C(4)(2+) and Ti(2+) at the mass-to-charge 24 Da.

Microanalysis of two creep resistant 9–12% chromium steels

Abstract The ferritic chromium steels P122 and P92 have been investigated using APFIM and TEM. The two steels are similar in composition with the exception of an addition of copper to P122, allowing a higher chromium content. The investigated materials were tempered at 770°C and isothermally aged at 600°C for times ranging from 0 to 10 000 h. The matrix and precipitates of type M 23 C 6 , MX and Laves phase have been analysed with APFIM. Steel P122 contains 0.9% copper and it was found that the matrix concentration of copper drops during ageing from 0.4% to an equilibrium level at 0.1%, which is in good agreement with previous thermodynamical calculations. No copper was found in M 23 C 6 , MX or Laves phase. Copper instead forms a separate phase. This phase was identified by TEM. During ageing the amount of tungsten in the matrix drops due to formation of Laves phase. This process is faster in steel P122 compared with steel P92, indicating an accelerating effect of copper on the nucleation of Laves phase. In both steels, enhanced concentrations of boron were found inside M 23 C 6 carbides. Phosphorous was found to segregate to a very narrow region at the carbide/matrix interface.

Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals: Part 1 – Effect of nickel content

Abstract The effects of increasing the nickel content from 3 to 7 or 9 wt-% were investigated in high strength steel weld metals with 2 wt-% manganese. Nickel additions were beneficial to strength but detrimental to impact toughness. Significant segregation of nickel and manganese to interdendritic regions was observed at the two higher nickel contents. In these weld metals a mainly martensitic microstructure developed at interdendritic regions, whereas bainite was found at dendrite core regions. The microstructural inhomogeneity was due to segregation and the accompanying stabilisation of austenite in solute enriched regions to lower transformation temperatures. With 3 wt-% nickel the microstructure was found to be more homogeneous, with mainly bainite forming. The decrease in impact toughness with increasing nickel content was mainly attributed to the formation of coarse grained coalesced bainite.

Influence of strain on precipitation reactions during creep of an advanced 9% chromium steel

Abstract Creep tested samples of the 9% chromium steel P92 were investigated using energy filtered transmission electron microscopy (EFTEM) and atom probe field ion microscopy (APFIM). The mean size and the volume fraction of precipitates of type M 23 C 6 , VN and Laves phase as a function of creep time and temperature were determined. A comparison with isothermally aged material showed that coarsening of M 23 C 6 carbides is accelerated by the strain, while the effect of strain on VN precipitates is insignificant. It was also found that the number density of Laves phase particles is higher in creep tested material compared to isothermally aged material. Possible mechanisms to explain these observations are discussed.