Precipitation Of Carbide Particles Research Articles

Microstructure and Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Carbon and Silicon Additions

The microstructure, hardness, yield stress, fracture strain, and brittle‐to‐ductile transition temperature of Fe‐Al alloys with Al contents of 12‐18 at.% Al, which are in the range of the so‐called K‐state with possible short‐range ordering reactions, and with ternary additions of carbon and silicon were studied with respect to the effects of possible impurities on the hardening of Fe‐Al alloys. It was found that perovskite‐type Fe3AlC carbide particles precipitate even in alloys with low C and Si contents; they are controlled by prior heat treatments and strongly affect the deformation behaviour.

Effect of Partial Solution Treatment on Toughness of 12%Cr-0.3%C Steels.

In 12%Cr-0.3%C martensitic steels, the effect of partial solution (PS) treatment in the (γ+carbide) two phase region was investigated in terms of the relation between microstructure and impact toughness. PS treatment is very effective for refining γ grains, because insoluble carbide particles works as obstacles to grain growth. The γ grain refining results in a significant lowering of Ductile-Brittle Transition Temperature (DBTT). This is mainly due to the reduction of grain boundary stress concentration which is caused by pile-up of dislocations to grain boundary. Besides, PS treatment contributes to homogeneous dispersion of carbide particles: In specimens with full solution (FS) treatment, carbide particles precipitate along lath boundary of martensite during tempering, and this causes early crack initiation in impact testing. On the other hand, in specimens with PS treatment, carbides preferentially precipitate on insoluble carbide particles which have homogeneously dispersed within the martensite matrix. This results in an increase of upper shelf energy in impact testing. It was also found that PS treatment suppresses precipitation of grain boundary carbide which occurs during slow cooling from γ phase region, because the insoluble carbide particles provide preferential precipitation site within γ grains. As a result, DBTT is not dependent on cooling rate in PS treated specimens, although it is greatly raised in FS treated specimens with slow cooling because of the grain boundary precipitation of carbide.

Bainite Transformation in a Silicon Steel.

The processes of upper bainite formation, the crystallographic aspects and the internal structure in a 1.83% silicon steel have been investigated. The T-T-T diagram is separated into two C-curves as in the case of steels containing strong carbide forming elements. In the early stage of upper bainitic transformation, very fine needlelike ferrite subunits with the parallelogram cross sections elongated in a γ // α direction form on a{111} γ sheet in a side-by-side fashion. In the later stage of transformation, these subunits are coalesced on a{111} γ plane and produces upper bainite laths. This bainite is always related to the parent austenite with the Kurdjumov-Sachs relationship and accompanies the surface reliefs. e carbide particles precipitate mainly after the ferrite subunits formation or after the coalescence of them. Therefore, the carbide precipitation is not a fundamental characteristic of bainite transformation but is a secondary effect. e carbide needles precipitating within a bainite lath by long time holding are aligned almost parallel to the α with almost constant intervals and are related to the ferrite by the Jack relationship. These aspects are exactly similar to those in tempered martensite. Such an e carbide configuration is thought to arise from the precipitation on the dislocations introduced during transformation.

Influence of plastic deforamtion by shot peening and of cyclic loading on the properties of the surface layer of 30KhGSN2A steel

1. The high effectiveness of repeated surface hardening, revealed in an increase in cyclic life of the high-strength steel, is caused by the high amount and depth of penetration of the first order residual compressive stresses (in comparison with the first peening) and by the significant changes in phase composition and fine structure of the surface layer of the material. 2. Cyclic loading of the material occurring before repeated peening intensifies the processes occurring in the surface layer in plastic deformation of it (decomposition of residual austenite and the martensite phase with precipitation of carbide particles, relaxation of local microstresses and a reduction in deformation nonuniformity of the structure, an increase in dislocation density). All of these processes promote an increase in the resistence to microplastic deformation and in fracture toughness and, consequently, lead to an increase in the resistance to the origin and development of fatigue cracks.

Threshold behavior of small fatigue crack at notch root in type 304 stainless steel

Fatigue crack growth of notched specimens in Type 304 stainless steel was investigated both at room temperature and elevated temperatures of 450 and 500°C. Continuous measurements of crack closure were made at elevated temperatures in this study. An accelerated crack growth rate and extra crack opening were observed for the small cracks initiated at notch root both at room and elevated temperatures. Non-propagating cracks were observed at 450 and 500°C, although they were not observed at room temperature under fully-reversed loading. The accelerated crack growth rate for the small cracks at notch root was well adjusted on the data for long cracks by using the effective stress intensity factor range, Δ K eff . The non-propagating crack was found to occur when the value of Δ K eff decreased and reached its threshold value. Precipitation of carbide particles was found to be less predominant for the crack growth at notch root in Type 304 stainless steel at elevated temperatures up to 500°C.

高合金オーステナイトステンレス鋼のクリープ破断強度におよぼす微量Ti, Nb添加の影響

Improvement of the creep-rupture strength of 18-8 stainless steel (Type 304) by additions of small amounts of Ti and Nb has previously been reported.Such effect of additions of Ti and Nb on the creep-rupture strength of high-alloy austenitic stainless steels such as 22Cr-12Ni or 25 Cr-20Ni has been studied.As a result, it was found that the combined additions of small amounts of Ti and Nb to high-alloy austenitic steels are also effective for improving the creep-rupture strength just like as the case of 18-8 austenitic steel.Further effects of additions of Ti and Nb were found to be as follows:(1) the shape and distribution of precipitated carbides change, (2) the carbide particles precipitate finely and are evenly dispersed. It is concluded that the improvement of creep-rupture strength is considered to be due to the improved dispersion effect of these fine carbide particles.

321および347型鋼の長時間時効後の組織および機械的性質の変化

18-8 series stainless steels, which being used widely for big boiler plants recently, reveal some changes ofmechanical properties during their use at high temperature, resulting fromthe changes of structures, i. e., precipitation of carbide particles and their coalescence.Type 321 and 347 stainless steels were aged for up to 10000 hr at approximate service temperature in useof 650°C. The changes of structure, especially, the changes of observed shapes of precipitated carbides, were examined by optical and electron microscopies and those of mechanicalproperties were also investigated.As results, precipitated carbides had many shapes, i. e., massive, fiborid, dendric, net-like and rod-likeM23C6 and spheroidal TiC on the grain boundaries and fine spheroidal TiC, needle-like TiC and fine spherroidalM23C6 within the grains in type 321 steel, and also massive and spheroidal carbides (NbC) onthe grain boundaries and spheroidal and massive carbides (M23C6+NbC), rosettes of NbC and needle-likecarbide within the grains in type 347 steel. The Charpy impact value at room temperature and the creeprupture strength at 650°C of both steels became lower owing to long time aging.

Electron microscope investigation of carbide precipitation and ferrite decomposition in steel Kh18G14 at elevated temperatures

1. Short-time heating at 600–800°C of quenched steel containing 18% Cr and 14% Mn results in the precipitation of carbide particles (M23C6) at the interface between the α and γ phases borders of γ phase growing from these particles into the body of the ferritic constituent. 2. Increasing the holding time at the above-mentioned temperatures assists in revealing the substructure of the ferrite and austenite, which may be due to the formation of carbon (and nitrogen) accumulations around dislocations forming the boundaries of the substructure blocks. 3. The guiding phase in the process of the (α→γ+σ) transformation must be considered to the the σ-phase