High-alloy steel, as a type of steel containing a high proportion of alloying elements, is widely used in modern industries that require high strength, corrosion resistance, and high-temperature oxidation resistance. Its unique chemical composition grants it excellent properties, but also presents a series of technical challenges during the forging process. Compared to ordinary carbon steel, high-alloy steel exhibits significantly different physical and mechanical characteristics during heating, deformation, and cooling, which directly influence the selection of forging processes.
The forging of high-alloy steel not only involves traditional mechanical deformation but also requires consideration of the material's phase transformation behavior, the influence of alloying elements, thermal conductivity, and potential cracks and defects during deformation. These factors demand precise control of temperature, speed, and equipment configuration during forging to ensure the structural integrity and performance of the final product.
This article will delve into the main characteristics and challenges of high-alloy steel during forging, analyze its phase transformation characteristics, forging temperature requirements, and forging performance, provide a series of technical points for optimizing the forging process, and summarize how to overcome potential technical difficulties in the production of high-alloy steel.
High-alloy steel typically contains a large number of alloying elements, which interact with the base metal to form different phases, influencing the material's properties. Common phases include:
Uniform phases: For example, austenitic solid solutions, which exhibit high plasticity and are suitable for high-temperature applications.
Strengthening phases: Such as dispersed carbides and intermetallic compounds, which help improve the steel's strength while having a minimal impact on plasticity.
Brittle phases: Such as the α-phase in austenitic steel and the γ-phase in ferritic steel, which significantly reduce the material's plasticity and make it prone to cracking.
Brittle non-metallic inclusions and coarse precipitates: Such as eutectic carbides in high-speed steel, which often increase the material's brittleness.
Low-melting-point phases: Such as iron sulfide and iron eutectics, which may melt at high temperatures, further affecting the material's stability.
In high-alloy steel, the above phases do not always coexist, but the first three are relatively common. The addition of alloying elements, especially the formation of complex phases, hinders atomic diffusion in the base metal, thereby increasing the recrystallization temperature and slowing down the recrystallization rate. For example, the recrystallization temperature of pure iron is about 450°C, while steel with 0.49% carbon shows obvious recrystallization at 750°C. However, heat-resistant steel containing 0.42% carbon, 15% chromium, and 7% nickel does not recrystallize even when deformed at 900°C.
Compared to carbon steel, high-alloy steel exhibits significantly different temperature characteristics during heating and forging. This is because high-alloy steel contains a large number of alloying elements, which alter the atomic arrangement of the steel, resulting in poor thermal conductivity. In particular, high-alloy steels containing large amounts of chromium and nickel have thermal conductivity coefficients much lower than those of carbon steel. At room temperature, the difference can be as much as 2.5 times, and this gap gradually narrows as the temperature increases.
The low thermal conductivity and plasticity of high-alloy steel necessitate slower heating rates, especially during the heating of large-sized ingots, where large temperature differences can easily induce thermal stress and cause cracking. Therefore, when heating high-alloy steel, appropriate preheating is required, and the furnace temperature should be kept low with a controlled heating rate.
Due to the complex composition of high-alloy steel, low-melting-point substances tend to form at grain boundaries at high temperatures, which may lead to the formation of δ-phase in some austenitic steels or excessive growth of ferrite grains. Given this characteristic, the initial forging temperature of high-alloy steel is usually lower than that of carbon steel, but its final forging temperature is higher. This is because high-alloy steel has a higher recrystallization temperature, stronger deformation resistance, and lower plasticity, so the final forging temperature cannot be too low.
Based on the above analysis, the forging temperature range of high-alloy steel is narrower compared to carbon steel. The forging temperature range of carbon steel is typically 350-400°C, while that of some high-alloy steels is only 100-200°C.
The forging performance of high-alloy steel differs significantly from that of carbon steel, mainly in the following aspects.
The deformation resistance of high-alloy steel at forging temperatures is several times higher than that of carbon steel or even ordinary alloy structural steel. For some high-temperature alloys, the deformation resistance can be as high as 5-8 times that of carbon steel. As the degree of deformation increases, the deformation resistance increases significantly, showing a strong tendency toward hardening.
During forging, especially during high-speed deformation (such as hammer forging), the deformation resistance of high-alloy steel is much higher than during low-speed deformation (such as press forging). This indicates that high-alloy steel requires greater force during high-speed deformation.
Compared to carbon structural steel, high-alloy steel has lower plasticity within the forging temperature range. For example, the allowable upsetting deformation for some heat-resistant steels is 60%, while for some high-temperature alloys, it is only 40%. This indicates that high-alloy steel has lower plasticity, and excessive deformation may lead to cracking.
High-alloy steel presents some special issues during forging, and the following points require particular attention.
Since as-cast structures are brittle, initial forging should involve light blows to preliminarily break the as-cast structure. As deformation gradually penetrates the interior, the force of the blows can be increased. If forging is performed on a flat anvil, the low plasticity of the ingot may lead to longitudinal cracks, so special care is required.
The main deformation processes should be carried out at higher temperatures, avoiding operations such as chamfering and punching at low temperatures. Low-temperature operations can easily cause cracking due to localized low temperatures. Therefore, all tools in contact with the billet should be preheated.
The surface condition of the ingot has a significant impact on the plasticity of high-alloy steel. After removing surface defects (i.e., "peeling") through mechanical processing, the reduction can be increased by 1.5-2 times without causing cracking.
High-alloy steel has unique forging characteristics and temperature sensitivity, requiring careful control of temperature, deformation rate, and plasticity during forging. A deep understanding of the phase transformation characteristics, heating, and forging performance of high-alloy steel can help us better select appropriate process parameters to ensure the quality and performance of the final product. Additionally, given the compositional complexity and special high-temperature behavior of high-alloy steel, adopting reasonable preheating, temperature control, and surface treatment measures is key to successful forging.
