The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstätten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstätten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.     

The kinetics of σ-phase precipitation in AISI310 and AISI316 steels

The kinetics of σ-phase precipitation in AISI 310 and AISI 316 steels were studied. An attempt was made to express the rate of precipitation of σ-phase with formal kinetic theory equations. It was established that rate of precipitation of σ-phase is a function of temperature mat agrees with the Johnson-Mehl equation.     

Deformation-induced microstructure and martensite effects on transgranular carbide precipitation in type 304 stainless steels

Plastic deformation of 304 stainless steel (SS) induces transgranular (TG) carbide precipitation, which is critically dependent on deformation-induced microstructural changes occurring during thermal treatment of the SS. Uniaxial deformation of the 304 SS to 40% strain produces a high density of intersecting micro-shear bands composed of heterogeneous bundles of twin-faults and about 12–17% strain-induced α′-martensite at the intersections of the twin-faults. Thermal treatment of 670°C for 0.1–10 h, however, results in a rapid annihilation/transformation of the strain-induced martensite and the concurrent formation of zones containing mixed thermal martensite laths and fine-grained austenite, though the thermal martensite also decreases with increasing heat treatment time. Simultaneous with these thermomechanically-induced microstructural changes, TG chromium-rich carbides form at intersections of twin-faults and on fine-austenite or thermal martensite boundaries in the SS; however, no correlation between strain-induced α′-martensite and carbides was observed in this work. The mechanisms of deformation-induced microstructure and (strain-induced and thermal) martensite effects on TG carbide precipitation in 304 SS are discussed.     

Static recrystallization of austenite and strain induced precipitation kinetics in titanium microalloyed steels

Using torsion tests and applying the back extrapolation method the recrystallized fraction of austenite was measured at two strains (0.20 and 0.35) and several temperature makes it possible steels. Graphical representation of the activation energy vs the inverse of the temperature makes it possible to determine the static recrystallization critical temperature (SRCT) of each steel as a function of the strain. With the results obtained, a model was constructed for the recrystallized fraction both at temperatures above and below the SRCT, and this, together with other equations already published, permits calculation of the residual strain and austenite grain size which the steel is expected to have at the end of rolling and before the γ → α transformation. Graphs of the recrystallized fraction vs time show the kinetics of strain induced precipitation and PTT diagrams (precipitation-time-temperature) were constructed for the three steels. It is noted that the degree to which the strain affects the precipitation kinetics decreases as the titanium content of the steel is increased.     

Influence of microalloy type and content on induced precipitation kinetics in microalloyed steels

Using torsion tests, and applying the back extrapolation method, the statically recrystallized fraction has been determined for low carbon microalloyed steels with different V, Nb and Ti contents, at different temperatures and an equivalent strain of 0.20%. At temperatures below the static recrystallization critical temperature (SRCT), or the temperature at which strain induced precipitation commences, recrystallized fraction against time curves show a plateau caused by precipitations. The formation of the plateau makes it possible to evaluate precipitation-time-temperature (PTT) diagrams, and thus to know the precipitation kinetics. The PTT diagrams show that an increase in the microalloy content raises SRCT and reduces incubation time. For similar Nb, V and Ti contents, Nb is the element which raises SRCT the most.     

The prediction of precipitation strengthening in microalloyed steels

The effectiveness of interphase precipitation strengthening in microalloyed steels depends on the temperature dependence of the solubility of the precipitation phase in austenite and on the temperature utilized in soaking. Using an approximate method of calculating the solubility of microalloying elements in the presence of both carbon and nitrogen, a precipitation strengthening potential parameter was developed. On relating this parameter to the chemical compositions and thermal histories of microalloyed steels, it was determined that the interphase precipitation strengthening determined in this and other studies increases linearly with the strengthening potential parameter. On the basis of the linear dependence of precipitation strengthening on the precipitation potential and other observations, it appears that interphase precipitation strengthening is not due to the Orowan looping mechanism but rather to interactions of gliding dislocations with the strain fields of coherent precipitates.     

Preparation technology of carbide steels

Kinetic dependences of the average particle size of carbides and steels during milling in an attritor are investigated. The volume content of various fractions in the powders under study is established. The technology of preparing the powder charges for the acquisition of carbide steels 40X2-B₄C, P6M5K5-TiC, and X18H15-Cr₃C₂ used in the production of machine building wares operating under conditions of increased wear, high temperatures, and corrosion media is presented.     

SANS and TEM studies of isothermal M2C carbide precipitation in ultrahigh strength AF1410 steels

Results are presented of SANS and TEM experimental investigations of the precipitation and growth, as a function of isothermal ageing time, of the main secondary hardening M₂C phase in ultrahigh-strength AF1410 steel. The results obtained are compared with APFIM investigations and with theoretical models. Combined use of these techniques provides valuable diagnostic data for quantifying the development of microstructures in this class of steels. It is found that the observed precipitation behaviour is consistent with the theory of precipitation at high supersaturations, but includes a surprising secondary burst of nucleation at precipitation half completion.     

The kinetics of hydrogen attack of steels

The hydrogen attack of steel involves the formation of methane bubbles along the grain boundaries and their subsequent link-up to form fissures. This paper presents a detailed model for the kinetics of growth of such methane bubbles. The model considers two parallel processes which can control the growth—one involving the bubble growth by direct power-law creep and the other involving combinations of surface diffusion, grain boundary diffusion and matrix accommodation processes. The proposed model is more general and complete than the earlier ones and considers for the first time the possibility of bubble growth being controlled by surface diffusion and accommodation processes. The predictions of the model are shown to compare well with the experimental results obtained in our lab and with the literature data. The model also indicates the relative importance of lower carbon activity and increased creep stength of steel to its hydrogen attack resistance.     

The crystallography of secondary carbide precipitation in high speed steel

Precipitation of secondary carbides in a powder metallurgical high speed steel, ASP 23, has been investigated by transmission electron microscopy. Particular attention has been paid to the crystallographic orientation relationships between the precipitated phases and the ferrite matrix. During the hardening treatment prior to tempering, cementite precipitates on prior-austenite grain boundaries and twin boundaries within the martensite plates. The cementite is oriented with respect to one of the adjacent ferrite lattices according to the Bagaryatskii orientation relationship. During tempering at 560°C the cementite coarsens and fine dispersions of MC and M₂C precipitate within the martensite plates. MC obeys the Baker-Nutting orientation relationship while M₂C obeys both the Pitsch-Schrader orientation relationship and another relationship defined by: (0001)_M2C//(021)_x; (11¯20)_M2C//(100)_x; (¯1100)_M2C//(01¯2)_x. The coexistence of both of these orientation relationships for M₂C precipitated in ferrite is explained in terms of the near-coincident site lattice model.     

On the kinetics of carbide precipitation during reaction between graphite and Fe-Ti liquids under microgravity

Experiments on the reaction between graphite and liquid Fe-Ti alloys were performed with a mirror furnace on board an airplane during parabolic flights. Small Fe-Ti alloy samples were melted in contact with graphite and held for some seconds at a temperature of 1550 °C. The samples were melted and solidified during a microgravity period. Carbon and titanium atoms reacted in the melt and titanium carbides were formed. In the experiments, a precipitation zone with faceted titanium carbide crystals dispersed in high carbon Fe-C-Ti alloy matrix was obtained near the graphite/alloy interface. The thicknesses of the carbide precipitation zones were measured and effects of alloy composition on the growth rates of the carbide zones were revealed by experiments and calculations. It was shown that the process was controlled by the diffusion of titanium in the liquid at low titanium concentrations and by diffusion of carbon through the precipitation layer at high titanium concentrations in the melt. Supersaturation of the carbide in front of the reaction interface was predicted from the calculations. The analysis showed that homogeneous nucleation of titanium carbide can readily occur in the alloys. Carbide morphologies were analyzed, and the mechanisms which lead to their formation are discussed.     

Indices of the serviceability of carbide steels

Translated from Poroshkovaya Metallurgiya, No. 2(326), pp. 48–53, February, 1990.     

Cracking resistance and strength of carbide steels

Conclusions The carbide steels with binders of austenitic or austenitic-martensitic steels have high cracking resistance which is not inferior to that of the tungsten carbide hard alloys (at the same volume content of the metallic phase). This can be explained by high ductility of the binder of these materials.There is a relationship between cracking resistance and certain properties of the carbide steels, such as hardness, proof stress, and the limiting plastic strain in compression: a reduction of HV, _0.1, and an increase of ₁ are accompanied by an increase of K_1c. Consequently, it is possible to evaluate the cracking resistance on the basis of these available and relatively easy to determine mechanical properties.Translated from Poroshhkovaya Metallurgiya, No. 1(325), pp. 90–94, January, 1990.     

Sintering kinetics of tantalum carbide

Conclusions Tantalum carbide sinters at a temperature above 2500C. Decreasing the powder particle size activates the sintering process, but even with a powder of 0.17-m particle size specimens sintered at 2700C have a porosity of 11%. Coarse powders (> 7–8 m) sinter, without densification, at 2000–2200C by a surface self-diffusion mechanism. Fine powders (<7–8 m) undergo densification already at temperatures above 1400C by a diffusion-viscous flow and a volume self-diffusion mechanism during long holding periods and also probably by an activated grain-boundary sliding mechanism in the initial stage of sintering after rapid heating.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 16–19, October, 1982.     

Splitting phenomena occurring in the martensitic transformation of Cr13 and CrMoV14 stainless steels in the absence of carbide precipitation

Previously unknown splitting phenomena were detected in the martensitic transformation of XCrl3 and XCrMoV14 stainless steels using high resolution dilatometric analysis. These splittings, which are denominatedM _S0 in this article, indicate the martensitic subtransformation of areas of austenite rich in carbon and carbide-forming elements. In contrast to other types of splitting known until now, theM _S0 occur in the absence of carbide precipitation during cooling. From the experimental results obtained in this study, it can be concluded that the splittings resulted from concentration gradients produced in the austenite as a consequence of the partial or total dissolution of M₂₃C₆ carbides during heating. Formerly with the Centre Nacional de Investigaciones Metalúirgicas (CENIM), C.S.I.C     

The lower bainite transformation and the significance of carbide precipitation

The nature and significance of carbide precipitation accompanying the lower bainite transformation in steels has been examined. It is found that lower bainitic cementite nucleates and grows from within supersaturated ferrite, and that such precipitation is strongly inconsistent with the concept of 'interphase precipitation' at the α/γ transformation interface. Crystallographic determinations suggest that the lattice invariant shear of lower bainite may not be directly responsible for the occurrence of the characteristic single carbide variant. It is suggested that it is the transformation strain associated with a lower bainite platelet that stimulates the development of a particular carbide variant which is compatible with the relief of this strain. The transition temperature from upper to lower bainite has been tentatively rationalised on thermodynamic grounds and in the context of the B_s and M_s temperatures. It seems that the appearance of epsilon carbide in some lower bainites but cementite in others can be understood in terms of a theory of martensite tempering due to Kalish and Cohen.