Increasing the service life of pellet car grate bars made of 40Х24Н12СЛ steel

Intercrystallite corrosion (ICC) is the main reason to decrease service life of pellet car grate bars made of steel 40Х24Н12СЛ. In the process of the study of microstructure of pellet car grate bars manufactured according to GOST 977–88, the structure defects were established that increase the tendency to ICC. To prevent the ICC of the bars, a mode of heat treatment was elaborated, consisting of two basic operations: air hardening with preliminary exposure within 80 minutes at the temperature 1100 °С, followed by tempering within 150 minutes at the temperature 900 °С. The study of the structure of the pilot grate bar showed that the selected mode of the hardening enables to dissolve the carbide network in the structure of the detail and crash large carbides, and the subsequent tempering contributes to the growth of the carbide phase in the entire volume of metal, partially remaining in the structure after hardening. It was shown that the proposed mode of the heat treatment contributes to bringing the structure of the metal to a more “equilibrium” state, which will allow to exclude the formation of large carbides and carbide network, growth and coagulation along the grain boundary. Results of tests of a pilot batch of grate bars installed at an industrial induration machine with a working area of 768 m2 and operating temperature of 20–1310°С presented. It was established that the elaborated technology ensures absence of casting defects in the grate bars – cracks and warping, inclination of the grate bars to ICC, increased by 23.13% heat resistance compared to the reference grate bars of foreign supply due to the globular form of carbide secretions with a minimum length of borders along which the metal matrix is depleted by chrome. Industrial testing of the grate bars manufactured under the experimental technology has proved the practical significance and applicability of the performed research.

Effect of Heat Treatment on the Structure and Properties of the Carburized Layer of the Legs of Three-Ball Drill Bits

The effect of time-temperature parameters of heat treatment on the structure and properties of carburized case and the core of 19CrMnNiMo steel was studied. The critical points were determined by dilatometric analysis: Ac1 = 740°C, AC3 = 835°C. It was established, that after carburizing at 940 °C, prequench to 890 °C with oil cooling, quenching at 790 °C and tempering at 180 °C, martensite structure of carburized case with uniformly distributed carbides and the least amount of retained austenite is formed. The hardness of carburized case decreases smoothly from the surface into the depth, in proportion to the decrease in the carbon concentration and amounts to 60-50 HRC. The technological process of heat treatment of drill bit legs made of 19CrMnNiMo carburized steel providing minimal amount of retained austenite in structure, absence of carbide network and combination of optimum mechanical properties which is proved by a real on-site experiment is developed. Temperature conditions of carburizing, quenching and low tempering are recommended for the production of legs of roller bits.

Study of factors contributing to decrease of carbide heterogeneity in bearing grades of steel

At present to meet requirement to the quality of bearing steels it is necessary of accomplish estimation of carbide heterogeneity by SEP 1520 and provide the level of carbide network, carbide liquation and streaking, not exceeding adjusted limits, required by customer. To estimate possibility to decrease carbide heterogeneity (segregation), factors influencing its value in the rolling production considered. Description of the process of long products of bearing steels production at the rolling mill 370/150 of the OJSC “BMZ - managing company of the holding “BMK” presented. Possibilities of the existing mill equipment considered for 804 accomplishing of technological operations aimed at decrease of carbide heterogeneity in the finished product. The factors studied, contributing to decrease of carbide heterogeneity under conditions of mill 370/150 operation. To determine the temperature modes effect on the level of carbide heterogeneity in bearing steels, in the process of three campaigns phased decrease of the temperature of end of rolling was accomplished. Results of data analysis obtained at the production of long products profiles of 34-50 diameter of bearing steel grades presented. Conclusion on rolled stock production of bearing steel grades with high requirements to carbide heterogeneity value was formed. It was shown that to obtain high-quality long products it is appropriate to accomplish the heating in the furnace before the rolling no less than 600 min at the temperature in the soaking zone 1150-1220°С followed by application of tech­nology of normalizing rolling and control of temperature of the end of rolling 750°С. For profiles with drawing less 25% it is rec­ommended to accomplish additional heat treatment of the finished profile - normalization. Long time heating of workpieces before the rolling at the temperature 1150-1220°С enables to decrease the carbide heterogeneity down to acceptable level (carbide liquation no more than 3 points and carbide streaking no more than 4 points). The value of carbide network is decreasing at decreasing of tem­perature of the end of the rolling and increasing deformation value in the last stands. A stable satisfactory result (CN no more than 5.4) was reached at the temperature of the end of rolling 750°С and drawing more than 25%. It was established that the higher the total drawing, the lower the point of carbide network and liquation in the finished profile, which is caused by crushing of nondissolved carbides to separate fragments.

Perfection of Fe–C–Si–Мn–Сr–W–V system flux-cored wire composition for surfacing of hot rolling mill rollers by introducing titanium in it

Rollers for hot rolling mills are hardened by surfacing operation by flux-cored wire ПП-Нп-35В9Х3СФ due to GOST 26101–84. The deposited layer has a high resistivity against abrasion, but its thermal endurance is comparatively low, therefore rollers surfaced by this type of wire often failed because of formation of fire crack network and spalling. It was established that the structure nonuniformity of the deposited metal can be decreased by introducing of titanium into the flux-cored wire. The effect of introducing titanium into flux-cored wire of the Fe–C–Si–Мn–Сr–W–V system on the properties of the deposited layer has been studied. It was shown that metal structure with the addition of titanium represents martensite formed within the boundaries of the former austenite grain, a small amount of residual austenite in the form of separate areas and thin layers of δ-ferrite. The microstructure of the samples contains a carbide network. An increase in the titanium content in the deposited layer contributes to a decrease in the size of the martensite needles, as well as the size of the former austenite grain. The microstructure of the samples contains medium-acicular and fine-acicular martensite. The size of the martensite needles varies from 2 to 9 microns. It was established that introduction of titanium in the composition of the flux-cored wire in an amount of 0.02–0.13% increases the hardness of the deposited layer and reduces the abrasion of the samples.

The Effect of Heat Treatment on the Oxidation Resistance of Cobalt-Based Superalloys

Degradation of the mechanical integrity of cobalt-based superalloys can occur as the carbide network is progressively oxidised during high temperature service. In this study, a heat-treatment aimed at redistributing the carbides was tested on two similar commercial Co-based superalloys, one with high C content (Co-101) and one with low C content (Stellite-21), to determine its influence on oxidation resistance. It was found that the carbide phases in the lower C-containing alloy could be solutioned more readily than the higher C-containing alloy, enabling the continuity of the carbide network to be reduced. This resulted in a reduced attack of the carbides down the interdendritic channels during oxidation testing, but increased thickness of the oxide overscale.

Investigation of the Effect of Short Exposure in the Temperature Range of 750–950 °C on the Ductility of Haynes® 282® by Advanced Microstructural Characterization

A Gleeble-based test method has been developed to study the change in the ductility signature of Haynes® 282® during isothermal exposure from 5 s to 1800 s. A temperature range of 750 to 950 °C has been used to investigate the effect of age-hardening reactions. Microstructural constituents have been analyzed and quantified using scanning and transmission electron microscopy. Carbides present in the material are identified as primary MC-type TiC carbides, Mo-rich M6C secondary carbides, and Cr-rich M23C6 secondary carbides. Gamma prime (γ′) precipitates are present in all the material conditions with particle sizes ranging from 2.5 nm to 58 nm. Isothermal exposure causes the growth of γ′ and development of a grain boundary carbide network. A ductility minimum is observed at 800–850 °C. The fracture mode is found to be dependent on the stroke rate, where a transition toward intergranular fracture is observed for stroke rates below 0.055 mm/s. Intergranular fracture is characterized by microvoids present on grain facets, while ductility did not change during ongoing age-hardening reactions for intergranularly fractured Haynes® 282®.

Determining the Optimum Conditions for Heat Treatment of Cold Rolling Mill Rolls by Studying Their Influence on the Microstructure and Properties of the Billet

Quality, reliability, and service durability of mill rolls are critical factors determining the quality of finished products and efficiency of rolling mills. The main problem that arises in the course of production of steel forged mill rolls is selection of reasonable conditions of heat treatment. As far as the size of the grain with an original structure and the pattern of distribution of separate structural components within the billet play a significant role in formation of final properties of the article, the objectives of this work are studying the heat treatment influence on the microstructure and hardness of steel mill rolls and development of reasonable process conditions on the basis of obtained data, in particular: determining the optimum conditions of primary heat treating the cold rolling mill rolls after their forging to reach the homogeneous structure and reduce the carbide grid score and carbide liquation; determining the optimum conditions of secondary thermal treatment of cold rolling mill rolls to provide the hardness within the range of about 300 HB, favorable structure for industrial frequency current hardening and reduction of carbide network score and carbide liquation; determining the hardening temperature, holding time and cooling rate to obtain the optimum structure and hardness within the range of 650-700 HB. Optimization of heat treatment conditions and analysis of steel microstructure have been performed with the application of cylindrical items (diameter 30 mm, height 20 mm) cut from the sample and template. Based on the study findings some microscopic images of steel microstructure after application of certain heat treatment conditions have been made and data on carbide network and current grain evaluation have been obtained

Experimental and ab initio study of the influence of a compound modifier on carbidic ductile iron

To improve the morphology of carbides in carbidic ductile iron, a compound modifier consisting of 0.1% Nb + 0.1% Ti + 0.1 wt.% Y was added to the base ductile iron with chemical composition of 3.72% C, 2.77% Si, 0.51% Mn, 0.99% Cr and balance Fe (wt.%). The effect of this compound modifier on the microstructures of carbidic ductile iron was studied. Also, first-principles calculations were carried out to better understand the modification mechanisms. The results showed that the maximum diameter of spheroidal graphite nodules decreased from 58 to 34 µm after the addition of compound modifier, and continuous carbide networks changed into a broken network. The roundness of graphite nodules decreased slightly, and the percent nodularity of the graphite nodules and the number of carbides decreased by 3 and 1.8%, respectively. Compounds with higher melting point are formed thanks to the compound modifier which acts as heterogeneous core, and the remaining Ti and Nb elements can be selectively attracted by (010) surface of Fe8Cr4C4. Furthermore, Cr elements can be easily replaced by Ti and Nb in the carbides to form more stable Fe8Cr3TiC4 and Fe8Cr3NbC4, which can prevent the continuing growth of carbide on the Fe8Cr4C4 (010) crystal surface and break the continuous network M3C. Y atoms cannot be directly adsorbed onto Fe8Cr4C4 (010) surfaces. They combine first with oxygen in the ductile iron to form Y2O3. The work of adhesion of the interface between a Y2O3 (100) and a Fe8Cr4C3 (010) is predicted to be 0.3 J/m2. The addition of Y element is found to have a positive effect on breaking up the continuity of the carbide network.