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Heat Treatments and Surface Hardening of Small Weapon Components
Published in Jose Martin Herrera Ramirez, Luis Adrian Zuñiga Aviles, Designing Small Weapons, 2022
Jose Martin Herrera Ramirez, Luis Adrian Zuñiga Aviles
If cooling is performed at a temperature just below 540°C (Figure 8.2), austenite transforms into bainite, a microconstituent whose microstructure consists of ferrite and cementite phases in the form of needles or plates, depending on the transformation temperature. Bainite is composed of a ferrite matrix and elongated particles of Fe3C. The times required for austenite to begin (Bs) and finish (Bf) its transformation into bainite increase, and the bainite becomes finer as the temperature decreases. The bainite formed just below 540°C is called upper bainite or feathery bainite, while that formed at lower temperatures up to 215°C is called lower bainite or acicular bainite.
The heat treatment of plain-carbon steels
Published in William Bolton, R.A. Higgins, Materials for Engineers and Technicians, 2020
In order to obtain this hard martensitic structure in a steel, it must be cooled quickly enough. The minimum cooling rate that will give a martensitic structure is termed the critical cooling rate. If the steel is cooled at a rate slower than this, then the structure will be less hard, because some of the carbon has had the opportunity to precipitate as cementite. Under the microscope, some dark patches will be visible amongst the martensite needles (Figure 12.1C), these being due to the tiny precipitated particles of cementite. The structure so produced is called bainite, after Dr. E. C. Bain, the American metallurgist who did much of the original research into the relationship between structure and rate of cooling of steels. Bainite is, of course, softer than martensite, but is tougher and more ductile. Even slower rates of cooling will give structures of fine pearlite.
Non-Equilibrium Diagrams and Microconstituents
Published in Joseph Datsko, Materials Selection for Design and Manufacturing, 2020
Bainite is the microconstituent or microstructure that results when a steel is isothermally transformed from austenite at temperatures between the minimum pearlite formation temperature and the start of the martensite transformation temperature. In appearance the microstructure of bainite is feathery, acicular, or needle-like and resembles martensite in this respect. On the other hand, bainite forms by a diffusion, nucleation, and growth process similar to the pearlite reaction and not by the “shear” mechanism of the martensite reactions. The differences between the pearlite reaction and the bainite reactions are simply which precipitates first, the ferrite or the cementite, and where. It is believed that in the pearlite reaction cementite precipitates first, followed by ferrite at the austenite grain boundaries, with growth perpendicular to the grain boundaries. In the bainite reaction it is believed that plates of supersaturated ferrite form along the (111) planes of the austenite grains and that particles rather than platelets of carbide begin to precipitate out of the supersaturated ferrite, with the rate of precipitation depending upon the temperature. Bainite is not normally found in plain carbon steels because the pearlite nose extends to very short reaction times, as shown in Figure 5–6.
Microstructure and properties of high power-SLM 24CrNiMoY alloy steel at different laser energy density and tempering temperature
Published in Powder Metallurgy, 2021
Miao Sun, Suiyuan Chen, Mingwei Wei, Jing Liang, Changsheng Liu, Mei Wang
The product of strength and plasticity is a comprehensive performance index that characterises the strength and toughness of metal materials. The higher the value, the greater the energy absorbed before the metal material is broken. In Table 5, the microstructure of 24CrNiMoY alloy steel fabricated by HP-SLM is mainly martensite. The product of strength and plasticity of 24CrNiMoY prepared at 120 J mm−3 reached 5.74 GPa%. After tempering, the microstructure has been transformed into tempered martensite. When the sample tempering at 500°C, the product of strength and plasticity reached 7.91 GPa%, an overall increase of nearly 37.8%. In the previous study, Zuo et al. [16] and Xi et al. [24] used laser cladding technology to prepare 24CrNiMoY alloy steel. Results showed that the microstructure is mainly bainite. The product of strength and plasticity is 6.8 and 5.52 GPa%, respectively. Therefore, the 24CrNiMo alloy steel prepared by HP-SLM in this study has good tensile properties after tempering. It shows great potential for application in the manufacturing of high-speed brake discs.
Effect of intercritical processing temperature on mechanical properties, microstructure and microhardness of ferrite - bainite medium carbon dual phase steels
Published in Cogent Engineering, 2021
Gurumurthy B M, Achutha Kini U, Sathyashankara Sharma, Pavan Hiremath, Gowrishankar M C
The enhancement of properties of medium carbon steels (AISI1040, 4140 and 4340) is possible if the dual phase structure consisting of ferrite and bainite is obtained during heat treatment. The dual phase is obtained by heating steels in the intercritical temperature range for partial austenitization and then rapidly quenching in a suitable medium (Caballero et al., 2001; Kumar et al., 2008; Sharma & Rajan, 2010) above room temperature for considerable time. The dual phase treatment is the controlled hardening process, which gives the two-phase structure of hard bainite embedded in the soft ferrite matrix (Sharma & Rajan, 2010). Promotion of bainite structure in steel increases the tensile strength and hardness with increased absorbed energy in impact failure. Growth of bainitic microstructure, unlike martensite, leads to provide a better combination of strength and ductility at high bainite content.
A study on the intergranular corrosion and pitting resistance of Inconel 625 coating by PTA-P
Published in Corrosion Engineering, Science and Technology, 2019
Raphael Amorim Lorenzoni, Ricardo Paris Gasparini, Ana Cláudia dos Santos, Temístocles de Sousa Luz, Marcelo Camargo Severo de Macêdo
The samples with smaller heat inputs, CD04 and HI110, presented the highest substrate hardness values. This can be explained by the lower temperature profiles achieved, increasing the cooling timing and consequently, promoting the formation of bainite and martensite. The specimen HI110 presented a high standard error due to the higher density of precipitates near the interface, as shown in Figure 2, interfering with the accuracy of the results. For higher dilutions (CD12) where grains presented a cellular growth [18] near the interface before turning into dendritic form. This causes a slightly lower microhardness due to the lack of precipitates in the grain boundary as in the next stages. This solidification mode is caused by a higher cooling rate because of the contact with the substrate, which increased the heat transfer rate. The cross-section micrograph for specimen CD12 can be seen in Figure 4 in a higher magnification obtained through SEM.