Carbide Coarsening in H13 Die Steel in Service
A hot work die is strong because it holds a fine dispersion of alloy carbides. Service takes that dispersion away. The particles grow, the spacing between them widens, and the yield strength falls with it. That is the softening that lets thermal fatigue cracking start, and it is written in the carbides. This page sets out the three carbides that matter in a 5 percent chromium hot work steel, what each one does at temperature, and how the austenitizing temperature decides how much carbide is left in the steel to begin with.
Why carbides decide how a die softens
Dispersion strengthening holds the yield strength of a tool steel up through the spacing between carbide particles. Hold the particles small and well spread, and the strength holds. Let them grow, and the spacing opens up and the strength falls. In a die casting die this is the whole of the softening. The temperature at the surface is high enough and the time is long enough for the carbides to coarsen, and once they have coarsened the surface can no longer carry the thermal stress without yielding. Chromium, molybdenum and vanadium all hold the coarsening back, and they do it in different ways and to different degrees.
Three carbides, three different jobs
The source examined the carbides in the softened band of a specimen cycled 15,000 times and analysed them by energy dispersive spectrometry. The largest carbides were rich in chromium. The smaller carbides were rich in molybdenum. The vanadium-rich carbide did not show up at all, because it holds a dispersion too fine to resolve. The three bands behave as the chemistry predicts, and the table below sets out what each contributes.
| Carbide | Where it comes from | How it behaves at temperature | What the EDS found |
|---|---|---|---|
| Chromium-rich carbide, Cr7C3 | From the chromium in solid solution, and detected at tempering temperatures as low as 500 C | The most susceptible of the three to growth, which is why the steel softens steadily between 500 C and 700 C | The largest carbides in the softened band |
| Molybdenum-rich carbide, M2C or M6C | Precipitates during tempering, and holds its size above 1000 F | Far less sensitive to growth than the chromium carbide, and it also slows the growth of the chromium carbide | The smaller carbides in the softened band |
| Vanadium-rich carbide, VC or V4C3 | The most stable of the three, and the only one that resists solution at 1010 C for an hour | Holds a very fine dispersion even at temperatures approaching 700 C | Not detected, because it stays too fine to resolve |
The three carbides in a 5 percent chromium hot work steel, the way each one forms, how each one behaves at service temperature, and how each one appeared in the EDS analysis of the softened band.
Austenitizing temperature sets how much carbide is left in the steel
Before a die ever runs, the heat treatment has already set how much carbide will be available to hold the strength. Raise the austenitizing temperature and more of the carbide goes into solution, where it strengthens the matrix and hardens the steel. Take it too high and the grain grows, with the loss of toughness that brings. The table gives the undissolved carbide left in the steel at three conditions, together with the hardness reached after quenching. The vanadium carbide is the stubborn one. Austenitizing at 1850 F for an hour dissolves the molybdenum and chromium carbides but leaves the vanadium carbide in place, and it takes the higher band to bring the rest into solution.
| Condition | Carbide that stays undissolved | Hardness after quenching |
|---|---|---|
| As annealed | Total carbide content about 4.4 wt percent | Not applicable |
| Austenitized at 1850 F for one hour | About 2.3 wt percent, mostly vanadium with some molybdenum | About 59 HRC |
| Austenitized at 1950 to 2000 F | About 1.5 wt percent, mostly vanadium carbide | About 61 HRC |
Undissolved carbide and quenched hardness against the austenitizing condition for H13. Most of the carbide goes into solution between the annealed condition and the lower austenitizing temperature, and the remaining vanadium carbide needs the higher band.
The coarsening law behind the loss of hardness
The rate of coarsening follows a diffusion law, in which the cube of the particle radius grows with the diffusion coefficient of the solute, the interfacial energy between particle and matrix, and the time at temperature. In a steel where cementite and an alloy carbide sit together, the cementite is always the coarser of the two and it coarsens faster than any alloy carbide. Among the alloy carbides of this grade the chromium-rich carbide is the one most prone to growth, and the presence of molybdenum and vanadium holds it back to a degree. Because chromium diffuses more quickly in ferrite than most metallic alloying elements, the chromium carbide is detected from as low as 500 C and coarsens quickly compared with the molybdenum and vanadium carbides. That is why a chromium hot work steel softens steadily between 500 C and 700 C unless molybdenum is there to slow it, and why the vanadium content is worth more to a die than its cost.
What this means on the job
The steel in these tests is premium grade H13, the chromium hot work steel covered on the H13 tool steel page. The softening this coarsening produces is on the H13 die softening page, and the resistance of the family to that softening on the hot work softening resistance page. The carbide types across the tool steel range are set out on the carbide types and hardness page, the volume fraction of carbide on the carbide volume fraction page, and the distribution of carbide through a bar on the carbide distribution page. The structure of the grade as quenched is on the H13 microstructure page, what the steel has to survive on the thermal fatigue cracking page, and the cooling design that keeps the peak temperature down on the die cooling line design page. How a grade is chosen for a press is on the high pressure die casting selection page.
Before you use this as an alloy specification
This page is a reference summary of a published study and it is not an Aobo Steel specification. The carbide figures and the heat treatment responses are reproduced from the source, which tested one grade under one set of conditions. The carbides a particular die carries also depend on the melting route, the analysis and the heat treatment it is given. Final specification is confirmed on the job.
Source, Effect of Design Factors on Thermal Fatigue Cracking of Die Casting Dies, D. Schwam, J. F. Wallace and S. Birceanu, Case Western Reserve University, final technical report to the U.S. Department of Energy, Award No. DE-FC07-00ID138486, October 2004.