Characterisation of the Microstructure of Plain Carbon Steel Formed during Isothermal and Continuous Cooling Following Austenitisation

The present study has been undertaken to compare the microstructure of the plain carbon steel, containing 0.65 carbon, which was formed during varying isothermal and continuous cooling conditions following austenitisation at the same temperature and soaking time. After austenitisation, one set of samples was subjected to isothermal treatment which was carried out at a temperature varying in the range of 650–400 °C, and the other one was continuously cooled to ambient temperature using different cooling rates ranging from 500 to 1.4 °Cs–1. The metallographic examination of the samples was fulfilled using light and TEM microscopy. Additionally, Vickers hardness measurements were performed.

Optimization of surface roughness and machining parameters for turning superalloy GH4169 under high-pressure cooling

Superalloys are important structural materials in the aerospace and petrochemical industries. Because of their excellent fatigue and oxidation resistance, superalloys are usually used predominantly in rotor and turbine components. The requirement of good surface quality and high-precision processes has been widely concerned. High-pressure cooling is a commonly used auxiliary processing technology in the metal cutting field, which can efficiently improve the quality of the processed surface. It is of great significance to study the surface roughness of GH4169 processed by cutting under high-pressure cooling conditions. First, the single-factor and orthogonal experiment methods were used. The single factors, interaction effect, and quadratic effect of cutting parameters and cooling pressure were analyzed. Then, the multiple linear regression method was adopted to establish the prediction model for surface roughness of GH4169, and the accuracy of this model was verified. Finally, according to the prediction model, the machining parameters were optimized and verified. The results indicated that the proposed model was accurate and reliable, which could be used to optimize the machining parameters. The optimal parameter combination was achieved, with the cutting speed of 154 m/min, the feed rate of 0.10 mm/r, the cutting depth of 0.46 mm, and the cooling pressure of 52 bar. Under this parameter setting, the prediction error was 4.20%, and the surface roughness was 34.92% less than that obtained under the actually recommended machining parameters. The above results will provide theoretical guidance for the parameter optimization of cutting superalloy under high-pressure cooling conditions and quality control of the processed surface.

Effect of Alloying Elements and Microstructure on the Dynamic Strain Aging Behavior of 1.25Cr-0.5Mo and 2.25Cr-1Mo Steels

The effect of alloying elements and microstructure on the dynamic strain aging (DSA) behavior of 1.25Cr-0.5Mo (P11, ASTM 335Gr.P11) and 2.25Cr-1Mo (P22, ASTM 335Gr.P22) steels was investigated. For both steels, different cooling conditions such as air-cooling (AC) and oil-quenching (OQ) were applied. Tensile tests were conducted in the temperature range of 20-450 ℃ and a strain rate in the range of 6 × 10-5- 6 × 10-3 s-1 for the steels with different cooling conditions. The P11AC steel showed serration behavior over a wider temperature range and exhibited higher ultimate tensile strength (UTS) than for the P22AC steel. This is attributed to the effects of alloying elements (Cr, Mo and Si) due to dissolved C, and the ferrite fraction on mechanical behavior. Meanwhile, the P11AC and P11OQ steels also showed different behaviors for DSA starting temperature, DSA temperature range, and serration type. The AC condition showed higher UTS from the interaction solid solution hardening (ISSH) effect due to substitutional Cr, Mo, and interstitial C elements. The calculated activation energy value (Q) for the P11 steel was around 94-103 kJ/mol-1, similar to that of ferritic steels, and it was higher for the P22 steel, with a Q value of 233 kJ/mol-1 from the ISSH effect.