1. Heat Treatment of Martensitic Corrosion-Resistant Stainless Steel Castings
Martensitic corrosion-resistant stainless steel cast steel contains 13% chromium by mass and a high carbon content, resulting in excellent hardenability. After proper heat treatment, it exhibits not only excellent overall mechanical properties but also good corrosion resistance. Therefore, this steel is often supplied in the heat-treated state.
The common quenching and tempering process for martensitic corrosion-resistant stainless steel cast steel involves oil quenching or air cooling at 950-1050°C, followed by tempering at 650-750°C. Tempering is generally performed immediately after quenching to prevent cracking in the casting due to quenching stress. The microstructure of the quenched and tempered state is tempered bainite and ferrite. ZG15Cr13 and ZG30Cr13 cast steels experience temper brittleness at temperatures between 300-600°C, so tempering in this brittle region should be avoided as much as possible. High-strength, low-carbon martensitic stainless steel castings, containing small amounts of alloying elements such as nickel, molybdenum, and silicon, exhibit excellent comprehensive mechanical properties, weldability, and wear resistance after normalizing and tempering. They are widely used in large turbine impellers, whether integrally cast or cast-welded. The typical heat treatment specification is 950-1050°C normalizing followed by 600-670°C tempering. For large castings such as turbine impellers, the heat treatment parameters (normalizing and tempering temperature, holding time, and cooling rate) must be strictly controlled to achieve the required casting performance.
2. Heat Treatment of Ferritic Stainless Steel Castings
Ferritic stainless steel typically contains 16% to 30% chromium. It undergoes no phase transformation upon heating and cannot be strengthened by heat treatment. Therefore, it can be used in the as-cast state for castings requiring less stringent corrosion resistance. Annealing can be performed to improve corrosion resistance and machinability. To avoid brittleness, the annealing temperature should be neither lower than 540°C nor higher than 850°C. After the annealing and holding period, the casting should be air-cooled or water-cooled; furnace cooling is not recommended.
Features of Annealing Heat Treatment for Ferritic Stainless Steel Castings:
1) Heating high-chromium stainless steel castings can easily cause grain coarsening, making the casting brittle and susceptible to intergranular corrosion. Therefore, overheating should be avoided.
2) Ferritic stainless steel castings are sensitized when rapidly cooled at temperatures above 850°C, causing intergranular corrosion. Therefore, the annealing temperature for ferritic stainless steel castings is best kept below 850°C.
3) Ferritic stainless steel castings, especially those containing 28% Cr, become brittle when heated to 700-800°C. Therefore, slow cooling after annealing can easily lead to brittleness. Heating at temperatures between 370-540°C can also cause brittleness.
3. Heat treatment of austenitic stainless steel castings
The cast state of austenitic stainless steel castings is a two-phase structure of austenite + carbide or austenite + ferrite. In order to make the steel have the best corrosion resistance, such castings generally need to be heat treated. Its heat treatment characteristics are:
(1) Solution treatment
The casting is heated to 950~1175℃, kept warm, and then quenched into water, oil or air to completely dissolve the carbides and obtain a single-phase structure. The choice of solution temperature depends on the carbon content in the steel. The higher the carbon content, the higher the required solution temperature.
The heating method for solution treatment of austenitic stainless steel castings should be to preheat at low temperature first, and then quickly heat to the solution temperature to reduce the temperature difference between the surface and the core of the casting during the heating process. The holding time is determined by the wall thickness of the casting. It is generally calculated as 1 hour for every 25mm wall thickness to ensure that all sections of the casting are heated through. The cooling medium for solution treatment can be water, oil or air, with water being the most common. Air cooling is only suitable for thin-walled castings. For austenitic stainless steel casts that cannot or are not suitable for solution treatment, a treatment process of keeping the steel at 870-980°C for 24-48 hours and then cooling it in air can be used to improve the corrosion resistance of the steel. However, this process is not suitable for thin-walled stainless steel castings with extremely low carbon content or castings that need to be welded after cutting.
(2) Stabilization treatment
Although austenitic stainless steel castings (18Cr8Ni type) have the best corrosion resistance after solution treatment, when they are reheated to 500-850°C or when the castings are used at this temperature, the chromium carbide in the steel will re-precipitate along the austenite grain boundaries, causing grain boundary corrosion damage or weld cracking defects. This phenomenon is called sensitization. To improve the intergranular corrosion resistance of this type of austenitic stainless cast steel, alloying elements such as titanium and niobium are generally added. After solution treatment, the steel is then reheated to 850-930°C and rapidly cooled. This allows titanium and niobium carbides to precipitate from the austenite first, thereby preventing the precipitation of chromium carbides and improving the steel's intergranular corrosion resistance when heated and used at these temperatures.
Post time: Sep-12-2025