How to control the casting state and metallographic structure of 45 # steel castings after heat treatment?

The metallographic structure of 45 # cast steel varies under different heat treatment conditions in the as cast state.

So, how do we control the casting state and metallographic structure of 45 steel castings after heat treatment when producing them? Fine control is required from both the casting process and heat treatment process, with the goal of obtaining a uniform, fine, and harmless structure to meet the final performance requirements (strength, toughness, hardness, etc.).

The following are key control strategies:

1、 Control the as cast microstructure (laying a solid foundation for subsequent heat treatment)

1. Optimize casting process parameters: pouring temperature: while ensuring the filling capacity, try to reduce the pouring temperature as much as possible. Excessive pouring temperature can lead to coarse grain size. The columnar crystal region expands. Increase segregation tendency. Promote the formation of the Wei organization. Cooling rate: Accelerate the cooling rate: This is the core of refining the as cast microstructure. A faster cooling rate can suppress grain growth, reduce segregation, and alleviate or even avoid the formation of Weibull structure. Method: Use metal molds or sand covered metal molds instead of pure sand molds; Place cold iron in the thick part of the casting; Optimize mold design (such as wall thickness uniformity and reducing heat savings); Choose modeling materials with good thermal conductivity; Control the temperature of the mold. Uniform cooling: Avoid significant differences in cooling rates between different parts of the casting, which can lead to uneven organization and internal stress. Reasonably design the pouring and riser system and the layout of the cold iron.

2. Inoculation/Modification Treatment: Although 45 steel is not conventionally inoculated like cast iron, in specific cases, it can be considered to add trace alloying elements (such as vanadium V, titanium Ti, niobium Nb) or rare earth elements for grain refinement treatment. These elements form high melting point compounds (such as carbides and nitrides) that serve as heterogeneous nucleation cores, promoting grain refinement. Accurate control of addition amount and process is required.

3. Control the purity of molten steel: Adequate deoxidation: Adopt reasonable deoxidation processes (such as precipitation deoxidation+diffusion deoxidation) to reduce the dissolved oxygen content in the molten steel, reduce FeO inclusions and the resulting grain boundary embrittlement. Common deoxidizers include manganese iron, silicon iron, and aluminum. Refining: If conditions permit, perform external refining (such as argon stirring) to further reduce gas (O, H, N) and inclusion content. Pure molten steel is beneficial for obtaining a denser, less defective, and uniformly structured as cast microstructure. Control the content of S and P: S is prone to form FeS or (Mn, Fe) S, forming low melting point eutectic at grain boundaries, increasing the tendency for hot cracking and deteriorating toughness; P increases cold brittleness. Efforts should be made to reduce the content of S and P to the lower limit required by the standard. 4. Mold design optimization: Reduce heat nodes and avoid prolonged exposure to high temperatures, which can lead to coarse grains and segregation. Ensure sequential solidification or simultaneous solidification to reduce defects such as shrinkage and porosity, which often result in abnormal microstructure in these defect areas.

2、 The conventional heat treatment for 45 steel cast steel parts is normalizing, and sometimes normalizing and tempering are carried out according to requirements to control the organization after heat treatment (the core is normalizing treatment). The purpose is to eliminate defects in the as cast microstructure and obtain a uniform and fine pearlite+ferrite structure.

1. Normalization treatment (most crucial):

Heating temperature: usually selected between 30-50 ℃ above Ac ∝. For 45 steel, Ac ∝ is around 780 ℃, so the normalizing temperature range is generally between 850-880 ℃. Purpose: To fully austenitize (gamify) the as cast structure, eliminate the original as cast structure (such as Weibull structure, coarse grains, and compositional segregation areas), and obtain uniformly composed austenite. Control: Low temperature, incomplete austenitization, residual as cast structure; Excessive temperature leads to significant growth of austenite grains, resulting in coarse microstructure after normalizing. Insulation time: It should be ensured that the casting is fully burned and the austenite composition is basically uniform. Calculation basis: Usually calculated based on the effective thickness of the casting (such as 1.5-2.0 minutes/millimeter). Control: Too short time, incomplete austenitization of the heart; If the time is too long, it may increase oxidation and decarburization, and the grain size may grow. For castings with dendritic segregation, it may take a slightly longer time for the components to diffuse evenly. Cooling method: Cooling in static or forced flowing air. Objective: To obtain finer pearlite (pseudo eutectoid structure) and finer ferrite grains than annealing. Control: The cooling rate should be uniform and consistent. Avoid: Cooling too quickly (such as too much wind): may cause a small amount of non-equilibrium structure (such as bainite or even martensite) to appear in the thin-walled area, increasing hardness and brittleness. Slow cooling (such as stacking too densely): loses the normalizing effect, and the structure coarsens and approaches the annealed state. Ensure that the castings have sufficient space outside the furnace for heat dissipation. The main function of normalizing is to eliminate coarse grains, columnar grains, and Weibull structure in the as cast microstructure. Refine grain size and achieve uniform structure. Eliminate internal stress (partially). Improve cutting performance. Provide better original structure for possible quenching and tempering in the future.

2. Annealing treatment

The metallographic structure of 45 # cast steel after annealing treatment is more uniform and stable compared to the as cast structure, mainly composed of the following parts: pearlite, which is the main component of the annealed structure and has a layered or sheet-like structure, composed of uniformly alternating ferrite and cementite. During the annealing process, the interlayer spacing of pearlite is more uniform and the distribution is more regular, which helps to improve the toughness and processing performance of the material. Ferrite: Distributed in block or small network form around pearlite or at grain boundaries. Compared with the as cast state, the annealed ferrite has a more regular morphology, more uniform quantity and distribution, reducing the adverse effects of coarse or networked ferrite that may exist in the as cast state on performance. The main function of annealing is to eliminate casting stress, refine grain size, and improve microstructure uniformity. Therefore, in the annealed 45 # cast steel structure, poor structures such as Weibull structure are basically eliminated, and the influence of casting defects (such as looseness) will also be weakened due to the densification of the structure. The overall performance is more suitable for subsequent processing or use.

3. Tempering treatment: For ordinary 45 steel castings, after normalizing, most performance requirements can usually be met without tempering. The cooling rate of normalizing is not sufficient to generate significant quenching stress. Situations requiring tempering: For castings that require extremely high dimensional stability, low-temperature tempering (150-250 ℃) can further eliminate residual stress. The casting structure is particularly complex, and there is excessive local stress during the normalizing cooling process (even if no martensite is produced). Improper control of the normalizing cooling rate leads to the appearance of a small amount of hard and brittle martensite or bainite in local areas, especially in thin-walled and sharp corners. Low temperature tempering (200-300 ℃) is required to reduce its hardness and brittleness. Tempering temperature: generally 150-300 ℃ (low-temperature tempering). Insulation time: Calculated by thickness (e.g. 1-2 hours/inch) to ensure heat penetration. Cooling: Air cooling. 3、 Control measures that run through the entire process 1 Strict composition control: Ensure that major elements such as C, Mn, Si, etc. are within the standard range (such as GB/T 11352 or ASTM A27/A27M). The fluctuation of carbon content directly affects the proportion and properties of pearlite and ferrite in the final structure. Strictly control the content of harmful elements S and P. Monitor the content of residual elements (such as Cr, Ni, Cu, Mo, etc.) to avoid their unexpected increase affecting the phase transition point and microstructure. 2. Metallographic inspection and feedback: As cast inspection: Sampling is taken at critical locations to check for serious issues such as coarse grain size, Weibull structure, and excessive non-metallic inclusions. Timely feedback is provided to adjust the casting process. Post heat treatment inspection: This is the most important step. After the final heat treatment (usually in the normalized state or normalized+tempered state), samples must be taken from the casting body or attached test block for metallographic examination: the microstructure type should be uniformly distributed fine pearlite+polygonal ferrite (sometimes ferrite is distributed in a mesh along the original austenite grain boundaries). It is not allowed to have residual cast structure, Weibull structure, a large amount of bainite or martensite. Grain size: Evaluate the austenite grain size grade (usually requiring 5-8 grades or finer). Non metallic inclusions: The rating is controlled within the qualified range. Performance testing: Cooperate with mechanical performance testing (tensile strength, yield strength, elongation, impact energy, hardness) to verify whether the organizational control achieves the expected performance goals. Summary of control points: 1. As cast foundation: low superheat casting+rapid and uniform cooling → obtaining a relatively small, uniform, and defect free as cast microstructure. 2. Core heat treatment (normalizing): precise temperature: Ac ∝+30~50 ℃ (850-880 ℃) → complete austenitization without growth. Sufficient time: thorough burning+uniform cooling of components; appropriate: uniform air cooling → obtaining fine pearlite+ferrite. 3. Necessary tempering: only used to relieve stress or treat local non-equilibrium structures (low-temperature tempering).

4. Pure ingredients: low in S and P, fully deoxygenated.

5. Strict inspection: The metallographic structure and mechanical properties of as cast and heat-treated materials are the final evaluation criteria.

By systematically controlling the above steps, it is possible to effectively ensure that 45 steel castings obtain ideal cast state and metallographic structure after heat treatment, thereby meeting their service performance requirements. **Metallographic examination is the ultimate means of verifying the effectiveness of all process controls.