Have you ever encountered situations where stainless steel castings corroded or fractured after just a few months of use? This is caused by inadequate solution heat treatment, which leads to carbide precipitation on the casting surface. This article will explain everything you need to know about solution heat treatment for stainless steel and help you understand its importance.
What is Solution Heat Treatment
Solution heat treatment is a heat treatment process that uses temperature changes to thoroughly dissolve internal impurities and precipitates in stainless steel, thereby restoring the metal to its optimal performance. It is commonly used in stainless steel castings.
Process
The solution heat treatment of stainless steel consists of three core stages, each of which is subject to strict data control. Understanding this process will help you identify the most critical points in solution heat treatment, giving you technical advantages when purchasing and enabling you to find the investment casting foundry that best suits your needs.
- Heating & Soaking
The heating and soaking stages mark the beginning of the solution heat treatment process for stainless steel. After casting, placing the metal castings, which retain significant internal stresses, in a heat-treatment furnace and heated to a temperature between 1050°C and 1150°C. Once the entire casting reaches the target temperature, the soaking time is adjusted based on the casting’s wall thickness to ensure that the precipitated chromium carbides are completely dissolved at high temperatures.
- Rapid Transfer
This step is critical to the success of the solution heat treatment, which is used to prevent carbide precipitation in stainless steel castings. After opening the heat treatment furnace, the hot castings must be immersed in the water tank within 10 seconds; for thin-walled parts, this must be done within 5–7 seconds.
- Water Quenching
After placing the castings in the tank, a high-power internal pump sprays them with high-pressure water to rapidly cool them down. This rapid cooling causes the microstructure inside the castings to solidify. However, it is important to note that the initial water temperature must be kept below 30°C, because if it is too high, rapid cooling cannot be achieved, and carbides will still precipitate.
The Importance of Solution Heat Treatment
Solution heat treatment offers significant benefits for stainless steel castings, including optimizing mechanical properties, eliminating residual internal stresses, and, most importantly, preventing intergranular corrosion. Understanding the importance of solution heat treatment for stainless steel castings will help you place greater emphasis on this process, thereby preventing casting fractures and the increased costs that can result from neglecting it.
Optimizing Mechanical Properties
Solution heat treatment can thoroughly homogenize and stabilize the microstructure of stainless steel castings, reducing hardness to the optimal range for cutting and ensuring smooth machining. For austenitic stainless steel, this range is typically between HB 160 and 190. Prevent issues such as tool wear and system breakdown during machining or use that can result from overall high hardness caused by internal microstructural inhomogeneity.
While reducing hardness, this process also significantly improves the casting’s toughness and ductility, enabling it to withstand high pressure during subsequent use and extending its service life.
Eliminating Residual Internal Stresses
In investment casting, the wall thickness of stainless steel castings is often uneven, as seen in pump housings, valve bodies, and pipe fitting castings. These uneven wall thicknesses can easily lead to nonuniform cooling rates, causing shrinkage in the casting and generating significant internal stresses.
Solution treatment can eliminate residual internal stresses during heating, maintain dimensional stability during subsequent machining, and prevent the casting from undergoing slight deformation during operation due to excessive internal stresses. This deformation could cause imperfect connection with other castings; for example, in piping fitting castings, if they cannot be connected to the main pipe casting perfectly, the entire system will be difficult to operate properly.
Eliminating Intergranular Corrosion
During the cooling process after pouring, carbon in stainless steel castings gradually diffuses outward, reacting with chromium, the key element for corrosion resistance, forming chromium carbides. These carbides precipitate at grain boundaries, leading to intergranular corrosion on the casting surface.
Solution treatment can thoroughly dissolve these carbides through heating, while subsequent water quenching freezes the internal microstructure, preventing the reformation of carbides. This ensures that chromium is uniformly distributed across the casting’s surface, allowing the chromium-rich passivation film to reappear and restore the casting’s corrosion resistance.
Common Defects Caused by Improper Solution Heat Treatment
Solution treatment offers many benefits, but improper solution treatment can also cause numerous problems. Being aware of these common defects will help you identify which issues are caused by improper solution treatment.
Uneven Hardness
During the heating and soaking phases of solution heat treatment, an aging heat treatment furnace or insufficient high-temperature soaking time can easily lead to excessive temperature variations within the furnace and incomplete solution treatment. This causes impurities, carbides, and hard phases on the casting surface to remain incompletely dissolved, resulting in uneven surface hardness. This can cause tool damage during machining and increase your costs.
Intergranular Corrosion
This is caused by a long water quenching transfer time during the solution heat treatment process. If the transfer time for a casting exceeds 10 seconds or the initial water temperature in the tank exceeds 30°C, the stainless steel will remain within the sensitization temperature range of 450°C–850°C. Within this range, chromium carbide precipitates will rapidly form on the surface of the stainless steel casting, leading to intergranular corrosion. If this is not detected in time, the casting will crack along the grain boundaries during operation, causing your system to make mistakes.
Casting Fragmentation
If a casting undergoes brittle fracture when water-quenched or subjected to a slight impact, it is because overheating occurred during the solution heat treatment, caused by the high temperature. Once the temperature exceeds 1,200℃, the structure inside the grain boundaries of the stainless steel will undergo slight melting, allowing oxygen to penetrate from the outside and causing intergranular oxidation. This directly damages the metal’s interior, turning all the castings in this furnace into scrap metal and significantly extending the lead time for your castings.
Oxidation and Decarburization
During the soaking phase of solution heat treatment, an excessively long soaking time causes the chromium on the casting surface to react violently with oxygen in the furnace, resulting in the formation of a thick, hard layer of metal oxide on the surface. This significantly increases the difficulty of subsequent pickling and polishing. This oxide layer also causes dimensional shrinkage of the castings and may even lead to surface decarburization, resulting in reduced hardness and worse wear resistance, which requires a significant amount of time for re-heat treatment. If the dimensional shrinkage is severe, the castings will be scrapped completely.
Water-Quenching Cracks
If you notice silvery-white or slightly yellowish glass-like cracks at sharp corners or stress concentration points on a casting, this is caused by violent thermal expansion and contraction inside the casting during water quenching. If the casting has abrupt severe wall thickness transitions or is immersed in water at an incorrect angle, the casting surface may crack immediately. Such castings have no compressive strength or air tightness. They must be recast, which will significantly increase your costs.
Solution Heat Treatment Process for Different Stainless Steels
There are many types of stainless steel, including austenitic, martensitic, and others. The methods for solution heat treatment vary depending on the type of stainless steel; some differ in temperature, while others differ in procedure. Understanding these differences will help you select the appropriate solution heat treatment method based on the material of your casting, thereby preventing the use of an unsuitable method that could result in impurities not being thoroughly removed from the casting’s surface and increase the cost of subsequent recasting.
| Stainless Steel Type | Common Cast Grades | Features of Solution Heat Treatment | Core Aims |
| Austenitic Stainless Steel | CF8 (304) CF8M (316) CF3 (304L) CF3M (316L) | Strictly control heating temperature between 1050°C and 1100°C; maintain a 10-second water quenching transfer time and a 30°C initial water quench temperature. | Completely dissolve chromium carbides, restore corrosion resistance, and optimize mechanical properties. |
| Duplex Stainless Steel | 4A (CD3MN) 5A (CE3MN) 6A(CD4MCu) | Demands higher heating temperatures and critical transfer time, that is, heating at 1100°C – 1150°C with a rapid 5 – 7 second transfer time. | Eliminate harmful sigma phases and perfectly balance the ferrite-to-austenite ratio. |
| Martensitic Stainless Steel | CA15 (410) CA40 (420) CB7Cu-1 (17-4PH) | Requires integrating solution heat treatment with subsequent thermal processing, typically Age Hardening or Quenching & Tempering | In contrast to austenitic stainless steel, it utilizes the precipitates to significantly increase hardness and tensile strength for castings. |
| Ferritic Stainless Steel | CB-30(431) CC50(446) | Insensitive to solution heat treatment; typically undergoes annealing treatment instead of water quenching. | Relieve internal stresses for casting and prevent embrittlement or cracking during operation. |
Critical Applications Requiring Solution Heat Treatment
Solution heat treatment is key to ensuring that stainless steel castings maintain their corrosion resistance and possess good mechanical properties. By understanding which applications of stainless steel castings require special attention to solution heat treatment, you can selectively apply this process based on your operating environment, thereby reducing unnecessary costs.
Marine Engineering
Seawater contains a large amount of chloride ions. After solution heat treatment, marine castings made of stainless steel can regain their corrosion resistance, enabling them to withstand pitting corrosion.
Furthermore, for castings such as pump castings, valve castings, impeller castings, or other castings which are used to transport seawater or other corrosive liquids, solution heat treatment helps reduce internal stresses in the castings. This can prevent pump and valve castings from experiencing slight deformation and even fluid leakage due to excessive internal stresses when subjected to fluid erosion.
Automotive Industry
In automotive castings such as turbocharger housings and exhaust manifold fittings, these components are particularly prone to developing thermal fatigue stresses due to repeated cycles of heating to 600°C to 900°C and cooling. Solution heat treatment can eliminate these internal stresses, thereby preventing high-temperature fatigue cracking in the castings during high-speed operation.
Chemical and Petroleum Engineering
Chemical engineering involves the transport of highly corrosive liquids such as strong acids and strong alkalis, and the media used in petroleum extraction often contain chloride ions. Solution heat treatment can thoroughly dissolve chromium carbides and diffuse chromium, which is the key element for corrosion resistance, to the surface of stainless steel casting, thereby greatly reducing the possibility of intergranular and crevice corrosion.
Furthermore, petroleum and chemical valves, such as high-pressure ball valves, operate under pressures in the hundreds of megapascals. Under the combined effects of external high pressure and internal fluid erosion, the castings themselves are highly susceptible to stress cracking. However, solution heat treatment eliminates internal stresses, effectively preventing micro-deformations and high-pressure leaks in the valve bodies.
Food and Pharmaceutical Engineering
Stainless steel castings that come into direct contact with food and pharmaceuticals must undergo repeated cleaning and disinfection during use, typically using acidic cleaning agents such as citric acid. If the castings themselves are susceptible to intergranular corrosion, these cleaning agents will corrode the surface, creating numerous tiny pits that can lead to bacterial growth. However, solution heat treatment can dissolve the chromium carbides that cause intergranular corrosion, which not only eliminates the possibility of such corrosion, but also prevents bacterial growth and ensures the castings remain hygienic.
International Standards of Solution Heat Treatment
Understanding international standards for solution heat treatment will help you specify the required standards for your castings to the manufacturer before casting begins, giving you a technical advantage and ensuring that the quality of the castings meets your expectations.
Currently, the most widely used global standards for solution treatment are the ASTM A351 and ASTM A890.
ASTM A351
ASTM A351 is the solution heat treatment standard for austenitic stainless steels, such as 304 and 316 stainless steel castings. This standard strictly specifies that the minimum heating temperature must reach 1040°C, the water quenching transfer time must be within 10 seconds, and the initial temperature must be around 30°C.
ASTM A890
ASTM A890 is the solution heat treatment standard for duplex stainless steel. This standard specifies a higher heating temperature, ranging from 1100°C to 1150°C, and emphasizes that the transfer time must be within 5–7 seconds, with quenching performed at a sufficiently fast rate to completely prevent the precipitation of harmful phases such as the sigma phase.
How to Verify Solution Heat Treatment Meets the Standards
There are many testing methods available to verify whether solution heat treatment meets the required standards. Understanding these tests allows you to determine whether castings meet the standards before they are put into operation, thereby preventing greater losses during subsequent operations.
Hardness Testing
The hardness of stainless steel after solution heat treatment should fall within a specified range; for example, the hardness of 316L stainless steel castings should be around HRB 80–90. The casting is securely placed on the hardness tester’s platform, and the hardness is measured at 3 to 5 points by rockwell hardness scale, with the average value calculated. If the measured hardness is too high, it indicates that the solution heat treatment did not completely remove the carbides from the casting’s surface.
Intergranular Corrosion Test
Intergranular corrosion testing is typically performed using the oxalic acid etching method, which is known as ASTM A262 Practice A. Test specimens are selected, polished to a mirror finish, and immersed in a 10% oxalic acid solution. A current of 1 A/cm² is applied, and the specimens are electrolytically etched for one minute. Then examined under a metallographic microscope. If the grain boundaries of the test specimens exhibit uneven, stepped surfaces without obvious black grooves, the test is considered satisfactory. If the grain boundaries are corroded into deep, black grooves, this indicates that the solution heat treatment has failed and the carbides have not been completely dissolved, thereby failing to eliminate intergranular corrosion.
Pitting Corrosion Test
This test is used to verify whether the solution heat treatment of duplex steel and 316L stainless steel castings meets specifications. Cut the test specimen into a 25 mm × 50 mm rectangular block and place it in a 6% ferric chloride solution. Depending on the critical pitting temperature, immerse the specimen for 24 hours; for duplex steel, the temperature is approximately 22°C–25°C, while for 316L stainless steel, maintained at approximately 0°C or 10°C.
After removing, cleaning, and drying the specimens, weigh them to determine the weight loss per square meter; this is typically required to be less than 4 g/m². Moreover, examine whether there is a dense network of pinpoint-sized pits on the surface under a microscope; their presence indicates that the solution heat treatment was unsatisfactory and that internal precipitates were not fully dissolved.
Final Thoughts
Solution heat treatment is key to ensuring that stainless steel castings can possess sufficient mechanical properties, excellent corrosion resistance, and a long service life. Hope you now have a thorough understanding of solution heat treatment. If you are looking for a stainless steel investment casting foundry with superior solution heat treatment processes, please contact us. Pingheng Machinery, with excellent quality control and rigorous solution heat treatment processes, will provide you with perfect castings.
FAQ
Q1. Why do 316L stainless steel castings rust after solution heat treatment?
The main reason is that the factory did not completely remove the chromium carbides from the castings during the solution heat treatment. It is also possible that the pickling and passivation of the castings did not meet the required standards, leaving iron ion contamination on the surface. If you want to know more about why stainless steel can rust, please click “Why does Stainless Steel Rust?”
Q2. How do you protect precision-machined casting from distortion during high-temperature water quenching?
We employ a scientifically designed pouring angle and use specialized fixtures to fix the castings. We also have sufficient material allowance from the outset to ensure that internal stresses are evenly and steadily distributed throughout the casting, preventing thermal expansion and contraction caused by sudden temperature changes.
Q3. Can we skip solution heat treatment to reduce stainless steel investment casting cost?
Yes. If the operating environment for your castings has no specific corrosion resistance requirements, you can skip this process to save costs. For example, purely residential castings or decorative castings, such as yacht railing bases and stainless steel furniture.
Q4. Is the cost of solution heat treatment included in your total cost of stainless steel castings?
Yes, we offer a one-stop service. From mold making to CNC machining to surface finish, we handle it all. Therefore, solution treatment, pickling, and passivation are included in our overall quote for stainless steel castings.
Q5. What is the risk of “Over-Soaking” during the solution treatment of ultra-low carbon stainless steel?
For ultra-low-carbon stainless steels, such as 316L stainless steel, the inherently low carbon content means that soaking at temperatures above 1050°C for too long can cause the austenitic microstructure to become abnormally coarse. This could significantly reduce the impact toughness of the casting under low temperatures and increase the risk of cracking during water quenching.
