It’s the nightmare of every stamping engineer: You’ve finalized the die design, the material is 304 stainless steel, and production starts. But after 50 hits, you hear the dreaded "crunch." The parts are tearing at the bottom radius.
Deep drawing stainless steel is notoriously difficult due to its high work hardening rate. Unlike mild steel, stainless steel gets harder and more brittle the more you deform it. If your process parameters aren't perfect, cracking is inevitable.
In this guide, you will learn:
- The physics behind why stainless steel cracks.
- 3 actionable solutions to fix tearing immediately.
- Real-world case study: Solving a 40% scrap rate issue.
Why Does Stainless Steel Crack?
The primary culprit is Work Hardening. As stainless steel (especially austenitic grades like 304 and 316) is drawn into the die, its crystal structure changes. It becomes stronger but significantly less ductile.
Cracking typically occurs in two zones:
Zone A: The Bottom Radius
The Problem: The punch nose radius is too sharp. The material is stretched too thin before it can flow into the wall.
Result: Circumferential tearing at the bottom.
Zone B: The Flange / Wall
The Problem: Excessive blank holder force prevents material flow. The cup wall cannot support the tension needed to pull the flange in.
Result: Vertical cracks or complete wall separation.
3 Engineering Solutions to Stop Cracking
Solution 1: Optimize the Draw Ratio (LDR)
Don't be greedy. Trying to draw too deep in a single hit is the #1 cause of failure. (See our cost comparison to understand why getting it right matters). For Stainless Steel 304, the Limiting Draw Ratio (LDR) is typically 1.8 - 2.0.
| Operation | Max Reduction (%) | Draw Ratio Formula |
|---|---|---|
| 1st Draw | 40% - 45% | Blank Dia / Punch Dia ≤ 2.0 |
| 2nd Draw (Redraw) | 20% - 25% | - |
| 3rd Draw | 15% - 18% | - |
*If your part requires a deeper draw, you MUST add an annealing step or a redraw station.
Solution 2: Switch to "Deep Draw Quality" (DDQ) Material
Not all 304 is created equal. Standard 304 might work for bending, but for deep drawing, you need 304 DDQ (Deep Draw Quality).
- Higher Nickel Content: Improves ductility.
- Controlled Grain Size: ASTM 7-8 grain size is ideal. Too large = orange peel; too small = poor formability.
ROI Impact:
DDQ material costs ~5% more but can reduce scrap rates from 20% to <1%.
Solution 3: Use High-Performance Lubricants & Coatings
Stainless steel has a high affinity for seizing (galling) to tool steel. Once galling starts, friction spikes, and the part tears.
The Fix:
- Tooling: Use TD (Thermal Diffusion) coated dies or Carbide inserts. (Learn more about surface finishing options).
- Lubricant: Use heavy-duty chlorinated oil or polymer-based dry film lubricants specifically for stainless.
Real World Case Study: The "Impossible" Filter Housing
Client: German Water Filtration Company
Part: 316L Filter Housing (Depth: 120mm, Dia: 50mm)
Issue: Previous supplier had 40% scrap rate due to vertical wall splitting.
Shengjie's Solution:
1. Redesigned process from 2 draws to 3 draws.
2. Changed material to 316L DDQ with specific grain size.
3. Applied TiCN coating to the redraw ring.
Don't Let Bad Design Kill Your Project
Download our Deep Drawing DFM Checklist to see the exact corner radii, clearances, and draw ratios we recommend for stainless steel.
Includes PDF with tolerance charts and material grades.