Formation of Nb(C,N) Carbonitride in Cast Austenitic Heat-Resistant Steel during Directional Solidification under Different Withdraw Rates

It is recognized recently that primary “Chinese-script” Nb(C,N) carbonitride is critical to the development of cast austenitic heat-resistant steels for ultra-high temperature applications. In this paper, the precipitation behavior of Nb(C,N) carbonitride in a novel creep and fatigue resistant steel was investigated by the use of the liquid metal cooling directional solidification (LMC-DS) method under different withdraw rates. Thermodynamic calculations were also performed to aid in the understanding of the solidification behavior. Microstructural characterization and thermodynamic calculation agreed that the alloy solidified in the path of primary austenite, eutectic Nb(C,N) carbonitride, and secondary ferrite, regardless of the withdraw rate. However, the primary and secondary dendrite arm spacing decreased significantly with an increase in the withdraw rate, and a quantitative relationship was established. Furthermore, the eutectic reaction range increased at a higher withdraw rate, due to the rapid increase of the solid phase fraction and the accumulation of solutes in the interdendritic liquid phase. This gave rise to a decline in the interlamellar spacing of primary Nb(C,N) carbonitride sheets and rods for the higher withdraw rate. Therefore, a fine “Chinese-script” Nb(C,N) carbonitride in this type of alloys can be achieved through increasing the withdraw rate or the cooling rate during casting.

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Effect of N/C Ratio on Precipitation Behavior of (Cr,Fe)23C6 Carbide in Novel Cast Austenitic Heat-Resistant Steels during Directional Solidification

Metals ◽

10.3390/met8090678 ◽

2018 ◽

Vol 8 (9) ◽

pp. 678 ◽

Cited By ~ 3

Author(s):

Yinhui Zhang ◽

Jian Yang

Keyword(s):

Directional Solidification ◽

Room Temperature ◽

Microstructural Characterization ◽

Eutectoid Reaction ◽

Precipitation Behavior ◽

Gasoline Engines ◽

Heat Resistant ◽

Δ Ferrite ◽

Liquid Metal Cooling ◽

Heat Resistant Steels

The precipitation of (Cr,Fe)23C6 carbide could significantly degrade the mechanical properties of Nb-bearing cast austenitic heat-resistant steels, designed for exhaust components of automotive gasoline engines at 1000 °C. In the current research, the precipitation behavior of (Cr,Fe)23C6 carbide in these alloys, with great variations in N/C (Nitrogen/Carbon) ratio, was investigated through the liquid metal cooling directional solidification method, combined with thermodynamic calculations. Microstructural characterization suggested that the (Cr,Fe)23C6 carbide formed in the steady-state zone and the competitive zone, upon cooling to room temperature, after the solidification ended. It grew in the colony of the δ-ferrite, through the eutectoid reaction and showed different concentrations of C and Si from the δ-ferrite. Its precipitation temperature decreased significantly with increasing the N/C ratio, thereby retarding its precipitation. Therefore, the quantity of (Cr,Fe)23C6 carbide could be limited though increasing the N/C ratio of this type of alloys.

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Quench Brittleness of 12%Cr Martensitic Heat-Resistant Steel

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.479.8 ◽

2011 ◽

Vol 479 ◽

pp. 8-12 ◽

Cited By ~ 1

Author(s):

Gang Yang ◽

Zheng Dong Liu ◽

Shi Chang Cheng ◽

Mu Xin Yang

Keyword(s):

Impact Toughness ◽

Cooling Rate ◽

Microstructural Characterization ◽

Temper Brittleness ◽

Resistant Steel ◽

Slow Cooling ◽

Continuous Precipitation ◽

Heat Resistant Steel ◽

Heat Resistant ◽

The Impact

The mechanism of brittleness due to slow cooling during quenching was experimentally investigated in 12% Cr martensitic heat resistant steel. The mechanical property tests and microstructural characterization by SEM、TEM and XRD were conducted. The results showed the impact toughness would decrease with the slowing of cooling rate during quenching, and the low cooling rate within the temperature range from 820 to 660 °C played a significant effect on the impact toughness . Different from the mechanism of temper brittleness, the main causes of embrittlement due to the slow cooling upon quenching were both the continuous precipitation of M23C6 along prior austenite grain boundaries during the process of slow cooling and that of M2C along prior residual austenite film during tempering, and this kind of quench brittleness was nonreversal.

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