Thermal Fatigue Resistant Tool Steels for Heat Checking and Hot Cracking
Thermal fatigue happens when a tool surface is repeatedly heated and cooled in service. The surface expands, contracts, and finally develops fine cracks. In hot work tooling, this failure is usually called heat checking, fire cracking, or hot cracking.
Thermal Fatigue Resistant Tool Steels Available from Aobo Steel
Aobo Steel supplies H-series hot work tool steels and tough hot-work support grades for die casting dies, hot extrusion dies, hot forging dies, mandrels, hot punches, and large hot-work tooling.
H13 | 1.2344 | SKD61
Balanced hot work steel for heat checking resistance, hot strength, toughness, and wear resistance.
H11 | 1.2343 | SKD6
Hot work steel with stronger toughness and cracking resistance for inserts, hot punches, and tools at higher crack risk.
H10 | 1.2365 | SKD7
Hot work steel direction for tools that need faster heat transfer and reduced surface temperature gradients.
H21 | 1.2581 | SKD5
Tungsten hot work steel for higher-temperature service where hot softening is more serious than thermal shock.
L6 | 1.2714 | SKT4
Toughness-oriented option for large forging dies, die holders, and heavy hot work support tools.
For most hot work tools, H13 and H11 are the main choices. H10 is useful when fast heat transfer is important. H21 is suitable for higher-temperature service where hot softening is more serious than thermal shock.
What Is Thermal Fatigue in Tool Steel?
Thermal fatigue is surface cracking caused by repeated thermal stress. The tool surface heats faster than the core when it contacts molten metal or a hot workpiece. During cooling, the surface contracts faster than the inner section. This repeated stress cycle creates fine cracks on the working surface.
The first visible sign is usually a network of fine surface cracks. If the steel cannot slow crack growth, these cracks deepen and may cause edge chipping, surface spalling, or larger cracks.
Recommended Thermal Fatigue Resistant Tool Steels
Thermal-fatigue-resistant tool steels are primarily hot-work tool steels. The best choice depends on whether the tool needs balanced performance, higher toughness, faster heat transfer, or stronger resistance to hot softening.
| Grade | Equivalent Grades | Main Selection Reason | Typical Use Direction |
|---|---|---|---|
| H13 | 1.2344 / SKD61 / 4Cr5MoSiV1 | Balanced resistance to heat checking, hot strength, toughness, and wear | Aluminum die casting dies, hot extrusion dies, hot forging dies |
| H11 | 1.2343 / SKD6 / 4Cr5MoSiV | Better toughness and cracking resistance than H13 | Die inserts, hot punches, tools with higher cracking risk |
| H10 | 1.2365 / SKD7 | Higher thermal conductivity than H13, useful for reducing surface temperature gradients | Hot extrusion tools, press hardening dies, mandrels |
| H21 | 1.2581 / SKD5 | Better strength retention at higher working temperatures | Brass extrusion dies, copper alloy tooling, high-temperature hot work tools |
| 1.2714 | L6 / SKT4 | Toughness-oriented option for large hot work tools | Large forging dies, die holders, heavy hot work support tools |
H13 is the standard starting point for many thermal fatigue applications. H11 is better when cracking resistance is more important than wear resistance. H10 is useful when the tool needs faster heat transfer. H21 is better suited to higher-temperature work, but it is not the best choice for severe, rapid cooling.
Why These Tool Steels Resist Thermal Fatigue
Thermal fatigue resistance comes from several material properties working together.
Hot strength helps the surface resist plastic deformation during heating. Tempering resistance helps the steel keep its hardness and strength during hot service. Toughness slows crack growth after heat checking begins. Thermal conductivity reduces the temperature difference between the hot surface and the cooler core.
The main alloying elements also support these properties. Chromium improves oxidation resistance and carbide stability. Molybdenum and tungsten improve hot hardness and resistance to softening. Vanadium forms stable carbides, which help maintain a finer structure.
H10 contains about 3% chromium, while H13 contains about 5% chromium. This gives H10 higher thermal conductivity, about 32 W/m-K compared with about 26 W/m-K for H13. In tools where surface temperature changes quickly, this can help reduce heat checking.
H21 works from a different direction. Its tungsten content helps the steel retain strength up to about 1150 F (620 C). This makes it useful for brass, copper alloys, and other high-temperature hot-work tools, provided the tool is not exposed to severe, rapid cooling.
Common Applications of Thermal Fatigue-Resistant Tool Steels
Thermal-fatigue-resistant steels are used in hot-work tools, where the working surface repeatedly contacts hot material and then cools.
| Application | Main Thermal Fatigue Risk | Common Grade Choice |
|---|---|---|
| Aluminum die casting dies | Heat checking from molten aluminum contact and repeated cooling cycles | H13, H11 |
| Magnesium and zinc die casting dies | Surface cracking under repeated thermal cycling | H13, H11 |
| Brass and copper alloy tooling | Higher working temperature and hot softening risk | H21, H10 |
| Hot extrusion dies | Long hot contact, pressure, and surface cracking | H13, H11, H10 |
| Hot forging dies | Thermal cycling combined with pressure and impact | H13, H11, 1.2714 |
| Hot punches and mandrels | Local heating and repeated contact stress | H10, H13, H21 |
| Press hardening dies | Fast heat transfer and repeated heating-cooling cycles | H10, H13 |
How to Choose the Right Tool Steel for Thermal Fatigue Resistance
Select the grade according to the visible failure pattern and working temperature.
| Failure Condition | Better Grade Direction |
|---|---|
| General heat checking in hot work tools | H13 |
| Heat checking plus higher cracking risk | H11 |
| Severe thermal shock and crack growth | H11 |
| Fast heating and cooling, with need for better heat transfer | H10 |
| Higher-temperature brass or copper alloy tooling | H21 |
| Large hot work tools with heavy impact | H11 or 1.2714 |
| Thermal fatigue combined with hot wear | H13 or H21, depending on working temperature |
| Thermal fatigue combined with hot softening | H13, H10, or H21 |
In short, use H13 for general balance, H11 for stronger cracking resistance, H10 for faster heat transfer, and H21 for higher-temperature service without severe thermal shock. 1.2714 is a supporting choice for large hot work tools where toughness matters more than maximum hot work performance.
Need Bulk Thermal Fatigue Resistant Tool Steel Supply?
Aobo Steel supplies H13, H11, H10, H21, L6 and other hot work tool steel round bar and flat bar for bulk industrial orders. Send your required grade, size, quantity, and application.