Have you ever found castings covered in cracks after receiving? This is caused by hot tears or cold cracks that form during the investment casting process. If not addressed properly, this can result in an entire batch being scrapped, project delays, or even customer refunds. This article will provide a detailed explanation of the main characteristics, causes, and preventive measures for hot tears and cold cracks, as well as the differences between the two.
What are Hot Tears
Hot tears refer to the sinuous, black or dark reddish-brown cracks that typically appear on the surface of austenitic stainless steel, resulting from the material’s inherent properties and design flaws. Gaining a thorough understanding of hot tears, including their characteristics, causes, consequences, and preventive measures, will enable you to address the issue with greater confidence and significantly reduce the time to find solutions.
Main Characteristics
- Shape
Hot tears occur during the final stages of metal solidification. It develops as the liquid film between grains, which has not yet solidified, is torn apart, resulting in a winding, discontinuous, jagged crack.
- Color
This color appears black or dark reddish-brown. This is because during the final stage of metal solidification, when a hot tear occurs, the casting is in a high-temperature crystallization zone exceeding 1000°C. After hot tears occur, the stainless steel is directly exposed to air, causing the surface to oxidize rapidly; the black or dark reddish-brown color visible on the surface of the hot tear is a thick layer of oxide film that has formed.
- Location
The locations where hot tears occur are almost always the same, that is, in the thickest central sections of castings, at cross-sections where wall thickness changes abruptly, and at sharp corners. This is because these areas cool the slowest and remain the hottest during metal solidification; that means they are the last places where the liquid film appears and also where internal shrinkage stresses are greatest.
Causes
The fundamental cause of hot tear is the formation of cracks due to excessive tensile stress during the late stage of metal solidification, but there are also other factors.
- Properties of Stainless Steel
Stainless steel is a high-alloy material that contains various alloying elements. During solidification, due to the differing melting points of these elements, a solid-liquid coexistence will exist. Some low-melting-point elements will form a layer of unsolidified, low-melting-point liquid film between the crystals. This film is extremely susceptible to tearing under internal tensile stress.
What’s more, if some low-melting-point elements like sulfur and phosphorus exceed the specified limits, it will significantly prolong the existence of this liquid film, making it more prone to hot tears.
- Casting Process
In the process of pouring, an excess of refractory material in the shell would cause the fired shell to become tightly wedged against the casting. Furthermore, because of the high linear shrinkage rate of austenitic stainless steel, it causes greater reverse tensile stress to develop when the shell is wedged against the casting compared to other metals. This is why hot tear typically occurs in austenitic stainless steel such as 304 and 316.
Moreover, if the pouring temperature is too high, it would cause coarse grain growth in the stainless steel, which not only affects the solidification time of the liquid film but also significantly reduces the internal tensile strength.
The combination of reduced internal tensile strength and increased external reverse tensile stress further increases the possibility of hot tear.
- Design
If the wall thickness of a casting is extremely uneven, there will be a significant time difference in solidification and cooling. The immense tensile force generated by the solidification and contraction of the thin walls will tear through the liquid film on the thick walls that has not yet solidified.
Furthermore, when a component design requires “T” or “L” shaped corners without smooth fillet transitions at the junctions, the tensile forces generated by metal contraction cannot be dispersed. This causes the grain boundary structure within the stainless steel to tear rapidly, leading to the formation of cracks.
Preventive Measures
To address the causes of hot tears described above, there are numerous targeted preventive measures. Understanding these preventive measures will enable you to prevent the risk of hot tears from the beginning of the casting design process, thereby avoiding the significant costs associated with scrapping castings and reworking entire batches later on.
- Maintain uniform wall thickness
When designing components, you could try to make the wall thickness of castings as uniform as possible. If the casting must include both thick and thin walls, you could ensure that the transitions between them are as smooth as possible.
- Avoid abrupt transitions
This is the most critical point: when designing your parts, you must absolutely avoid right-angle transitions and use sufficiently large fillets for sharp corners, that is, to reduce the possibility of internal tearing during casting shrinkage.
- Improve Shell Collapsibility
When manufacturing mold shells, the factory can strictly control the amount of refractory material used. When the stainless steel begins to be taken out, the mold shell will crumble slightly, preventing it from clamping down tightly on the casting and thereby eliminating external mechanical resistance on the casting.
- Optimize Pouring Parameters
During pouring, the foundry can employ a low-temperature, slow-pouring process. This prevents alloy elements with low melting points from clustering together over large areas, thereby avoiding the formation of extensive liquid films and significantly improving the tensile strength of stainless steel castings during the final stages of solidification.
- Scientifically design gating system
Foundries can design the gating system to be located at the thickest part of the casting, allowing solidification to proceed from the edges toward the center. Optimizing the gating system design ensures that the casting’s solidification shrinkage proceeds from the far to the near side, with the tensile stresses generated during this process concentrated in the riser.
This reduces the possibility of hot tears caused by tensile stresses resulting from shrinkage within the casting.
What are Cold Cracks
Unlike hot tears, cold cracking refers to the smooth, metallic-lustered fine lines that appear on the surface of castings made from martensitic stainless steel due to excessive internal stress. Understanding the main characteristics, causes, consequences, and preventive measures of cold cracking will help you distinguish it from hot tears and determine how to prevent it.
Main Characteristics
- Shape
Cold cracks are an instantaneous fracture, similar to the cracks that form in glass when it is struck. The internal stresses generated directly cut through the crystal structure; the cracks propagate through the metal structure, which is a phenomenon known as transgranular cracking, resulting in a straight, smooth, fine line.
- Color
Cold cracking occurs after the casting has completely cooled; therefore, the surface does not exhibit the oxidized blackening typical of hot tear, but instead displays only the smooth, silvery-white luster of the metal itself.
- Location
Cold cracking is a type of fracture caused by excessive internal stress; therefore, it primarily occurs at stress concentration points in castings, such as sharp corners, the edges of blind holes without chamfers, and the edges of CNC-drilled holes. In more severe cases, it can split the entire casting.
Causes
Cold cracks are primarily caused by the release of internal stresses within castings. Understanding the causes of cold cracks allows you to better grasp what preventive measures target, thereby improving your ability to prevent cold cracks.
- Uneven Cooling Rate
Stainless steel castings are often complex in shape, typically combining thin-walled and thick-walled sections. As the casting cools, the thin-walled sections lose heat more quickly and shrink first. This causes it to become tightly constricted within the mold shell, effectively preventing the thick-walled sections, which cool later, from shrinking. Consequently, significant thermal stress builds up inside the casting.
- Sudden Volume Change
Martensitic stainless steel would undergo microstructural transformations that convert austenite to martensite under 200℃. This is the primary reason why Martensitic stainless steel is prone to cold cracking. When this structural transformation happens, it is accompanied by significant volumetric expansion. Because the expansion of thin-walled and thick-walled sections of the casting occurs at different rates, with some sections expanding while others contract, enormous “microstructural stress” is generated internally.
- Improper heat treatment
At this stage, the casting already contains significant internal stresses, such as thermal stresses and microstructural stresses, making it prone to cracking at any moment. If the casting does not undergo proper stress-relief heat treatment to eliminate these internal stresses, the residual stresses will become a ticking time bomb. This can cause cracking during operation, which is the primary cause of delayed cold cracking in castings.
Preventive Measures
Cold cracking is primarily caused by internal stresses, particularly thermal stresses and microstructural stresses; therefore, preventive measures focus on eliminating these internal stresses. Understanding these measures allows you to prevent cold cracking during the design period, thereby reducing the time and financial costs.
- Adding Fillet Radii
Cold cracking is caused by internal stresses, which often concentrate at sharp corners, which are known as stress concentration points. Therefore, it is essential to add fillet radii at these corners during the initial design period. This ensures that internal stresses are evenly distributed across the curved surface.
- Uniform Cooling
Before pouring, foundries can wrap a layer of refractory insulation around the outer surface of the mold shell in thin walls to artificially slow down the cooling rate in those sections. Additionally, the pouring system can be optimized by positioning the sprue in the thin-walled area; this raises the temperature of the thin walls and extends their cooling time. As a result, the entire casting cools uniformly, significantly reducing the thermal stresses generated as the casting cools internally.
- Sand-Buried Cooling
Martensitic stainless steel undergoes a structural transformation when the casting cools to 200°C. Therefore, to reduce these stresses, manufacturers must place the freshly poured, hot mold shells directly into a heat-retaining pit or bury them in a pile of high-temperature quartz sand or ash powder immediately after pouring. By utilizing the high ambient temperature, the castings cool uniformly as a whole. This ensures there are no temperature differences between components, allowing the entire casting to expand simultaneously during the transformation, thereby eliminating the significant structural stresses caused by asynchronous cooling.
- Stress-Relief Annealing
To prevent sudden cold cracking in castings during subsequent operations due to internal stresses, manufacturers must perform stress-relief annealing immediately after the castings have cooled completely. This involves placing the stainless steel castings in a heat treatment furnace, heating them to 550°C–650°C, then holding them at that temperature for 2–4 hours, and cooling them slowly. This process reduces residual internal stresses within the casting to a safe level, which is close to zero, significantly preventing delayed cracking.
Hot Tears vs Cold Cracks
Hot tears and cold cracks differ significantly in terms of material sensitivity, location, and causes. Understanding these differences allows you to clearly identify which type of crack has appeared on a casting, enabling you to significantly reduce time.
Compare hot tears with cold cracks from a comprehensive perspective
| Comparison Perspective | Hot Tear | Cold Crack |
| Material Sensitivity | Often appears in austenitic stainless steel castings, such as 304 and 316L stainless steel castings | Often appears in martensitic stainless steel castings or precipitation-hardening stainless steels, such as 410 and 17-4PH stainless steel castings |
| Location | The hot spots, cross-sections with drastic changes in wall thickness, and sharp corners | Stress concentration points such as sharp dead corners, or directly penetrating the entire casting. |
| Main Characteristics | Tortuous and irregular cracks that appear black or dark reddish-brown | Straight and smooth cracks that are bright with a metallic luster |
| Causes | Excessive tensile stress in the casting, such as a low-melting-point liquid film being torn under mechanical hindrance stress | Excessive internal stress in the casting, such as thermal stress by uneven cooling rate & phase transformation stress by sudden volume change from austenite to martensite |
| Time of Occurrence | Occurs immediately at the end of metal solidification | Has delayed characteristics; sometimes occurs after the casting has completely cooled, but most of the time will suddenly crack during low-temperature operation |
Compare hot tears with cold cracks from Preventive Measures
- In the way of designers
In this regard, whether dealing with hot tears or cold cracks, they can be prevented through the same design methods. First, adding smooth fillet radii to sharp corners within the casting, which could reduce damage from tensile and internal stress to some extent.
Moreover, maintaining uniform wall thickness could ensure that there is no excessive time difference in cooling between thin and thick walls. This prevents the thin walls from shrinking too rapidly, thereby avoiding the tearing of the liquid metal film and its impact on the shrinkage of the thick walls, while simultaneously reducing the occurrence of both hot tears and cold cracks.
- In the way of foundries
Because hot tears and cold cracks occur at different times, there are different preventive measures for foundries to address them.
To prevent hot tears, foundries must focus primarily on measures taken during the pouring process, including controlling the refractory content of the mold shell and optimizing the pouring system, which includes controlling pouring temperature and moving riser placement.
However, to prevent cold cracking, the primary focus is on ensuring the casting cools uniformly as a whole through various methods, such as wrapping it in a layer of refractory insulation and using buried sand cooling. Furthermore, due to the delayed characteristics of cold cracking, foundries must perform an additional stress-relief heat treatment to prevent it.
Final Thoughts
Trust you now have a thorough understanding of hot tears and cold cracks, and are familiar with the measures to prevent them. If you are looking for a foundry that can effectively address these issues, please contact us. As a one-stop stainless steel investment casting foundry, Pingheng Machinery offers superior quality control and professional prevention and inspection methods to provide you with stainless steel castings that are completely perfect.
FAQ
Q1. If I find a crack on my stainless steel casting, what is the quickest way to distinguish whether it’s a hot tear or a cold crack?
Look at the color inside the crack. If the crack appears black or dark reddish-brown, it is a hot tear. If it shines with a bright, metallic, silvery-white luster, it is a cold crack.
Q2. Why do 304 and 316 stainless steel castings rarely experience Cold Cracking?
Since 304 and 316 stainless steels are austenitic stainless steels, they permanently maintain a stable austenitic structure internally and do not generate significant internal stresses due to structural transformations. Furthermore, they exhibit excellent toughness and ductility at low temperatures, making cold cracking hardly happen.
Q3. What is the minimum fillet radius required to prevent Hot Tears in stainless steel castings?
To prevent thermal cracking caused by sharp transitions, you need to design smooth fillets for the transitions. According to DFM design principles, the fillet radius (R) should be at least 0.5 to 1 times the thickness of the adjacent wall, and 90°sharp corners must be strictly avoided.
Q4. Can the cracks on the stainless steel castings be repaired by welding?
It depends on the application and standards for the casting. If it is a high-pressure valve casting, pump casting, pump housings, impellers, valve bodies, or a safety-critical component, the cracks caused by hot tears are not limited to the surface but extend into the interior of the casting. Once welding is performed, the high-temperature thermal stresses generated by the process will further cause the casting to crack again. Once a casting develops cracks, it must be scrapped without exception. However, for standard industrial components, once it has been confirmed through grinding that the cracks have been completely removed, rigorous preheating and welding repair followed by post-weld heat treatment may be performed.
Q5. Do you detect internal cracks in investment castings?
Yes, we utilize both liquid penetrant testing for stainless steel castings and X-ray inspection to ensure there are no internal cracks.
Q6. The levels of sulfur and phosphorus in stainless steel can affect the risk of hot tear and the corrosion resistance of castings. How do you ensure the contents of the two are strictly controlled within safe limits?
We use specialized spectrometers to conduct tests both before and after casting. This ensures that the sulfur and phosphorus contents are just right, keeping sulfur below 0.015% and phosphorus below 0.020%. This approach reduces the risk of hot tears while maintaining the castings’ corrosion resistance.
Q7. Except for hot tears and cold cracks, what other defects can occur in stainless steel investment casting?
Other defects include casting shrinkage and casting porosity. If you want to learn more about the types of defects that can occur, please click “Common Investment Casting Defects: Causes, Identification, and Prevention Methods”.
