Tool and Die Design Faults That Cause Cracking
A tool that cracks in the quench or breaks after a short run is often reported as a heat treatment problem, and the report is often wrong. Sharp corners, holes placed for convenience, stamp marks, heavy sections beside light ones and inserts that were never seated properly all put stress into the tool before it sees a load, and a heat treater cannot take those out of the part in front of him. This page lists the design faults that come back most often, what each one does to the tool, the change that removes it, and the corner radius data behind the fillet rule. It is written for designers, toolmakers and buyers who are asked to sign off a drawing.
Why the design is the first place to look
Tooling cannot be designed on dimensions alone. The designer has to know whether the tool will be quenched in water, oil or air, where the geometry puts light sections next to heavy ones, and how the part will be machined before and after hardening. Dimensional stability belongs in the same conversation, because a tool that moves in the quench cannot be brought back to size by the final machining. A tool that cracks during hardening is a design failure as often as it is a furnace failure, and the heat treater who is blamed for it was asked to harden a shape that could not be hardened successfully.
The other half of the job is planning. The design has to leave enough time in the route for the tool to be stress relieved, preheated, hardened, quenched and tempered properly, because the schedule is where most of these faults turn into scrap. A working relationship with the heat treater is worth more than any single line on the drawing.
The design faults that come back most often
Six faults account for the great majority of tools that crack early. Each one is visible on the drawing before anything is cut, and each one has a change that removes it without losing the function of the tool.
| Design fault | What it does to the tool | The change that removes it |
|---|---|---|
| Sharp corners and sharp-cornered keyways | Internal and external corners and keyways act as stress intensifiers. A sharp corner is the most frequent cause of premature tool failure and can raise the local stress to ten times the average calculated stress. | Leave a well rounded radius in every corner and use generous filleting wherever the function of the tool allows it. Every drawing should be checked for sharp corners that the tool does not actually need before it is released for manufacture. |
| Light sections next to heavy sections | The light section cools and hardens first during the quench, while the heavy section beside it is still hot. The differential cooling and transformation stresses that build up can pass the ultimate strength of the steel and produce distortion or cracking. | Where the shape is unavoidable, choose an air hardening steel so the whole part transforms at a rate the section can take. |
| Badly placed holes | Blind holes, threaded holes and holes placed without thought leave thin and weak walls between the hole and the edge of the die. Those walls see the same differential cooling stresses and are difficult to quench uniformly, especially in a liquid quenched steel. | Place the holes as part of the design rather than on the drawing after it. Where the hole pattern cannot be improved, choose an air hardening steel. |
| Stamp marks | A stamped impression is a sharp change of section cut into the surface. Stamps concentrate both the residual stress from heat treatment and the service stress, and a stress crack often runs straight through the characters. | Use low stress stamps with rounded characters, spread the marks out, and keep the number of characters down. Engraving with an electric pencil, paint, labels or etching is the better choice. |
| Rough machining | Heavy machining marks are built-in notches and they become the nucleation sites for cracks that open during hardening or in service. | Finish the surfaces that will see stress. This is the fault where the machinist and the die maker have to be told what the drawing means by a finished surface. |
| An insert that is not seated | The die insert touches its holder at the edges only, so the load is carried on part of the area. The uneven pressure and the deflection that follows open a crack at the corner of the insert. | Machine for full contact on the seating face. Where the dimensions do not allow it, machine undersize, true the tool up, and add a shim of the thickness the seating area needs so the contact is complete. |
Design faults behind premature tool failure, with the correction for each, from the tooling design chapter of Tool and Die Making Troubleshooter.
Corner radius and the impact strength it buys
The rule about fillets is easier to hold to when the size of the effect is on the table. The data below compares corner sections of the same tool with different radii at the base of the notch. The wider the radius, the stronger and more crack resistant the section, and the largest part of the gain comes early. Moving from a sharp corner to a 0.020 in. radius quadruples the projected impact strength, while the difference between 0.080 in. and 0.125 in. is worth very little.
| Radius at the base of the notch | Projected impact strength | What the change buys |
|---|---|---|
| 0.002 in. (0.05 mm) | 4 ft-lbf (5.4 J) | The sharp corner. The tool is at its most brittle at the notch root. |
| 0.010 in. (0.25 mm) | 7 ft-lbf (9.5 J) | A radius that a file could be said to have left. The gain over the sharp corner is already most of what a larger radius will give. |
| 0.020 in. (0.51 mm) | 16 ft-lbf (21.7 J) | A radius a machine shop would call small. Impact strength is four times the sharp corner. |
| 0.040 in. (1.02 mm) | 19 ft-lbf (25.8 J) | A radius a shop would call normal on a die corner. |
| 0.080 in. (2.03 mm) | 22 ft-lbf (29.8 J) | A generous fillet, the kind drawn where the designer has thought about the load path. |
| 0.125 in. (3.18 mm) | 24+ ft-lbf (32.5+ J) | The widest radius in the table. Beyond this the returns are small against the section the radius costs. |
Approximate strength comparison of corner sections with different size radii, from Tool and Die Making Troubleshooter. The source prints this comparison to demonstrate the effect of notch radius and states that it is not to be used for strength projections. Actual impact strength also depends on the grade, the heat treatment and the surface finish.
The comparison is not a promise about a specific die, and the source says so. What it does show is the direction and the size of the effect, which is enough to justify an argument with a drawing that carries a sharp internal corner for no reason. Where the part is loaded in impact, the grade itself is a separate decision, and the impact toughness chart and the Charpy impact test page carry the values by grade and hardness.
Grain direction against the direction of draw
Steel is like wood in one respect that matters to a drawing. It resists breakage better against the grain than with it, so a die that is loaded along the direction of draw will resist cracking better when the grain flow of the bar runs parallel to that direction. Dies that cracked through a stamped area have been examined and found to have their grain flow perpendicular to the draw, which is a layout decision made when the block was marked out from the bar. Replacing such a die with the grain direction turned and the deep stamps removed has produced a large improvement in the same application. How the bar is produced and what that does to grain flow is described on the wrought bar defects page.
Seating an insert so the load is carried everywhere
One cause of early cracking at the corners of a die insert is seating, and it has nothing to do with the steel. The bottom face of the insert has to make contact with its holder over the whole area. Where only the edges touch, the load is carried on a fraction of the face, the insert deflects, and the crack opens at the corner that took the load first. The same happens with a shim that has been added to the wrong place, which is why a shim is not a repair for a badly fitted seat.
Two routes are open where the dimensions do not allow full contact. The first is to machine the seat undersize, true the tool up until it sits correctly, and then add a shim of the thickness that covers the whole seating area rather than part of it. The second is to anneal the tool, weld to produce a buildup, reheat treat and remachine to dimension, which is the route taken when the seat itself has worn. Both are cheaper than the die that cracks after a shift. The assembly route for a fitted insert, including the interference a shrink fit needs, is on the shrink fitting page.
When the shape cannot be changed, change the steel
Some tools have to be drawn with the feature that makes them hard to harden, and where that is the case the material is the remaining lever. An air hardening steel transforms at a rate the whole section can follow, which removes the differential between a heavy section and the light section beside it. A shock resisting steel carries more toughness than a high-chromium cold work grade at the same hardness, which is the change made when a D2 punch holder or die section has cracked in service and the replacement has to keep working. Which of the two applies depends on whether the failure came from the quench or from the load, and that is a question about the fracture rather than about the drawing.
The grade families and the hardness each one will carry are tabulated on the tool steel properties chart and the composition chart. The specific substitution from a cold work to a shock resisting grade is described on the cracking resistance page and the chipping resistance page.
How this page sits next to failure analysis
This page is written for the stage before the tool is made, and the tool and die failure analysis page is written for the stage after it has failed. That page starts from the evidence on the part and works back to a cause by asking when the crack appeared, whether the surface is damaged and what the part was doing when it broke. This page starts from the drawing and lists the shapes that put stress into a tool before it runs. The overlap is real and deliberate, and the two are read together. Where the failure is a fatigue crack, the starting points and the notch and fillet effects are covered on the fatigue failure page, and the damage type itself can be narrowed on the damage mode identification chart.
Where to go next
The defects a wrongly designed tool shows in the furnace are listed on the heat treatment troubleshooting chart, and the size change a tool takes through the quench is on the size change page. Where the crack was opened by grinding rather than by the quench, the grinding cracks page covers the same surface damage from the other direction, and the wear that decides the rest of the die life is set out on the wear coefficients and design rules page.
Using this page on a drawing
This page is a reference summary of published practice and it is not an Aobo Steel specification. The design faults and the corner radius comparison are reproduced from Tool and Die Making Troubleshooter, and the source prints the comparison to demonstrate the effect of notch radius rather than to project the strength of a tool. The steel grade, the hardness and the heat treatment for a tool are confirmed on the drawing and the order for the job, and the design decision belongs to the person responsible for the tool.
Source, Tool and Die Making Troubleshooter, R. M. Leed, Society of Manufacturing Engineers, 2003.