How to produce high-quality high chromium cast iron parts?

High chromium cast iron is an extremely important wear-resistant material widely used in industries such as metallurgy, mining, cement, and power. Its melting and heat treatment processes require strict requirements to ensure obtaining ideal microstructure and excellent wear resistance.

The following is a detailed explanation of the key points of melting ingredients, melting temperature, pouring temperature, and heat treatment process for high chromium cast iron.

1、 The chemical composition of melted high chromium cast iron is the basis of its performance, usually with Cr/C (chromium carbon ratio) as the core design element.

1. Core chemical composition range (typical): Carbon (C): 2.0% -3.5%. The carbon content determines the quantity, morphology, and hardness of primary carbides and eutectic carbides. The higher the carbon content, the higher the hardness, but the toughness decreases. Chromium (Cr): 12% -30% (commonly found in 15% -28%). Chromium is a key element for forming carbides and ensuring the corrosion resistance of the substrate. The key point is to control the Cr/C ratio. Molybdenum (Mo): 0.5% -3.0%. Molybdenum can improve hardenability, inhibit pearlite transformation, and promote the formation of bainite or martensite, especially for large section castings. At the same time, it can refine the organization, improve toughness and wear resistance. Copper (Cu): 0.5% -1.5%. It is also used to improve hardenability and is a partially inexpensive substitute for molybdenum, but its effect is not as good as molybdenum. Nickel (Ni): 0-1.5%. Assist in improving hardenability and strengthening the matrix. Manganese (Mn): 0.5% -1.0%. Stabilize austenite and improve hardenability. However, excessively high levels can stabilize austenite, leading to an increase in residual austenite and segregation at grain boundaries, which is detrimental to toughness. Silicon (Si): 0.3% -1.0%. Deoxidizing elements, but will promote carbide graphitization, so the content should not be too high. Sulfur (S) and phosphorus (P): Strictly limited. P < 0.06%，S < 0.05%。 They are all harmful elements that can seriously reduce toughness and strength, and increase the tendency for thermal cracking.

2. The importance of Cr/C ratio: Cr/C<4: (Fe, Cr) ∝ C carbides will appear in the structure, with lower hardness and poor wear resistance. Cr/C ≈ 4-10: high hardness (Fe, Cr) ₇ C ∨ eutectic carbide (which is the main source of wear resistance of high chromium cast iron) is formed in the form of rod or strip, which has less splitting effect on the matrix and better toughness. This is the most commonly used interval. Cr/C>10: A large amount of (Cr, Fe) ₂ ∝ C ₆ - type carbides begin to form. Although the corrosion resistance is improved, the hardness decreases and the wear resistance is not as good as (Fe, Cr) ₇ C ₆.

3. Ingredient calculation: Calculate the furnace charge ratio based on the target ingredient and recovery rate. The furnace charge is usually composed of pig iron, scrap steel, chromium iron (such as high carbon chromium iron, low carbon chromium iron), molybdenum iron, copper, nickel plate, etc. Reference for recovery rate: Elements such as Cr and Mo have a high recovery rate when melted in a medium frequency induction furnace, usually calculated at 95% -98%. The recovery rate of Mn is about 85% -95%.

2、 Melting temperature and pouring temperature

1. Smelting temperature: The tapping temperature should not be too high, usually controlled between 1480 ° C and 1520 ° C. Reason: Excessive temperature can increase the burning loss of alloy elements (such as Cr and Si oxidation), intensify the absorption of hydrogen and nitrogen in the steel liquid, and make the grains coarse. Low temperature is not conducive to alloy melting, homogenization of composition, and slag iron separation.

2. Pouring temperature: The pouring temperature should be determined according to the wall thickness and structure of the casting, usually ranging from 1380 ° C to 1450 ° C. For thick and simple parts, a lower pouring temperature (such as 1380 ° C to 1420 ° C) should be used to facilitate sequential solidification, reduce shrinkage, and refine grain size. Thin walled and complex parts: Use higher pouring temperatures (such as 1420 ° C-1450 ° C) to ensure good filling ability. Principle: Under the premise of ensuring filling, try to use a lower pouring temperature as much as possible.

3、 Key points of heat treatment process

The as cast microstructure of high chromium cast iron is usually austenite+eutectic carbides+partial pearlite, with low hardness and poor toughness. A martensitic matrix with high hardness and wear resistance can only be obtained through heat treatment.

The core of heat treatment is "austenitization+quenching".

1. Austenitizing: Temperature: 940 ° C-980 ° C. The specific temperature depends on the composition, especially the content of Cr and C. For high carbon and high chromium formulas, take the lower temperature limit, otherwise take the upper temperature limit. Insulation time: Usually calculated based on wall thickness, insulation takes 1 hour for every 25 millimeters. Ensure that the carbon and alloying elements in the carbides are fully dissolved into the austenite, but prolonged time can lead to grain growth and carbide coarsening. Key point: After austenitization, the matrix becomes austenite rich in carbon and alloying elements.

2. Quenching: Cooling method: After being removed from the austenitizing temperature, it must be rapidly cooled (quenched). Common method: Air Quenching: This is the most commonly used and safe method. Due to its high alloy content and good hardenability, air cooling is sufficient to avoid pearlite transformation and obtain a martensitic matrix. For large or complex components, air cooling can effectively reduce the risk of cracking. Forced Air Quenching: using a fan to blow air and accelerate cooling. Oil quenching: Only used for very small or simple shaped castings, with high risk and easy cracking, requiring great caution. Purpose: To supercool high-temperature austenite below the martensitic transformation temperature (Ms point) and transform it into high hardness martensite.

3. Tempering: Necessity: After quenching, the internal stress is extremely high, and the structure is martensite+residual austenite, which is very brittle and must be tempered immediately. Temperature: Low temperature tempering is usually used between 200 ° C and 300 ° C, and sometimes medium temperature tempering around 450 ° C is also used (which reduces hardness but improves toughness). Insulation time: 2-6 hours (depending on wall thickness). Function: Relieve quenching stress and prevent cracking during use. Transforming quenched martensite into tempered martensite slightly reduces hardness, but significantly improves toughness and stability. Promote the transformation of some residual austenite into martensite (secondary quenching).

4. Special process: Subcritical treatment. For some working conditions that require high impact toughness, subcritical treatment with long-term insulation (such as 4-10 hours) between 450 ° C-520 ° C can be used. This process decomposes residual austenite into bainite ferrite and carbides, resulting in an excellent combination of strength and toughness, but the hardness may decrease.

Summary: A typical heat treatment curve for KmTBCr26 high chromium cast iron is as follows: [Austenitization] Heating to 960 ° C ± 10 ° C ->Holding for 4-6 hours ->[Quenching] Air cooling to room temperature ->[Tempering] Immediately heating to 250 ° C ± 10 ° C ->Holding for 4-6 hours ->Air cooling after discharge. Important reminder: Before entering the furnace for heat treatment, the castings must be cleaned thoroughly (removing molding sand, risers, etc.). The heating rate should not be too fast, especially for complex components. It is recommended to heat up step by step (such as maintaining a uniform temperature of 600 ° C for a period of time). After tempering, it must be cooled to room temperature before use. Only by precisely controlling the composition, melting, and a series of heat treatment parameters can high-performance high chromium cast iron wear-resistant parts be produced.