Deep-drawing stainless steel and nickel alloys means balancing many opposing factors. Adjusting hardness values to suit process requirements aids the decision-making process.
A print comes into the stamping plant for a deep-drawn part made in a familiar grade of stainless steel. No problem. Until you notice that the strength specification of the finished piece is quite a bit above what is normally produced in the process.
A specification calling for a tensile strength of 175,000 and a yield strength of 135,000 psi translates into three-quarter-hard material. That’s much too hard for forming. In stainless steel, the preferred range for incoming material is between the annealed condition and one-half-hard.
The Ideal Material
Stampers, understandably, want ductile materials that easily cold form. But specifiers of high-performance alloys usually don’t consider formability-they want strong, hard and tough parts. The ideal is a material that bends easily during forming but doesn’t bend at all once it’s a part. That’s a tall order. But with a little help from two under-appreciated factors – work hardening and heat treatment – stainless steel can approach that ideal.
Forming occurs somewhere between the yield strength and tensile strength of the material. If yield isn’t exceeded, forming doesn’t occur, but exceeding the tensile strength results in material fracture. In higher-strength materials, the window between yield and tensile is very small. It’s almost impossible to achieve desired formability and required tensile strength in the same material without taking some extra steps.
Usually, repeated press action as a part travels through a progressive die induces enough cold working to bring the material to quarter-hard or half-hard, often sufficient. When such cold working does not bring desired hardness, stampers have a couple of options. They can beef up tooling and choose a press that’s large enough to cold form a harder and stronger material. Such an option, besides its high cost, may result in part tearing or fracture as well as wear and tear on the tooling and press. There are always tensile tests performed on the material which can be a good indicator of how it will do with a deep draw test.
A better option may be forming the parts first and heat treating them later to elevate hardness and strength. For this option, an alloy must be selected with two hardness values in mind: ductile enough for cold forming and hard enough to meet finished part specifications. Hardness is the proxy for strength.
With high-performance alloys, hardness isn’t the only consideration. The buyer usually wants corrosion resistance, elevated temperature properties and other attributes. The job is finding an alloy within that specification that can climb the hardness scale to whatever value the buyer needs. To minimize cost, this should be done in the fewest numbers of passes through the rolling mill and furnace at the material supplier, and a minimal number of stamping stations at the metal forming operation. Of course, this can sometimes depend on thickness as well.
Quarter-hard, half-hard, full-hard, and spring-temper-hard (also referred to as extra-full-hard) are achieved by rolling specific percentage reductions on annealed materials. Hardness values stated here are actual specifications, not just rules of thumb – they are covered by an ASTM designation that refers to specific tensile-strength levels.
Quarter-hard is nominally 125,000 psi minimum tensile strength; half-hard is 150,000 psi, three-quarter-hard is 175,000 psi and full-hard is 185,000 psi. You’ll find a 25,000-psi spread between each figure except in the case of the three-quarter and full-hard, where the data converge because the work-hardening curve flattens out.
Yield minimums also exist for these hardness values. At Ulbrich Stainless Steels, we use 75,000, 110,000, 135,000 and 140,000 psi, respectively. Pulling sample parts on a tensile tester is the preferred method for measuring these properties. But Rockwell hardness testers are much more common in stamping plants than tensile-test equipment, so specification, Rockwell C numbers are useful, especially for 301 and 302 stainless, but aren’t very accurate for alloys with lower work-hardening rates.
The 300 series of austenitic stainless steels can only be hardened by cold working – heat treating is not an option. Because cold working takes place within the plastic range between yield strength and tensile strength, a look at a table of properties might suggest that Type 301 would be a good candidate for stamped parts because its range is comparatively wide. This grade cab handle much pulling and stretching but has a tendency to work-harden quickly. For that reason, 301 is not recommended for the deep drawing process.
Type 305 exhibits a much narrower range between yield and tensile strengths but is the preferred grade for deep-drawn applications. About 90 percent of stainless deep-drawn parts are produced from this grade. Because of its relatively high nickel content, work hardening increases very slowly during the forming process. It can be drawn over a series of does without becoming extremely hard or brittle and extensive drawing usually is possible before annealing is required. The good initial elongation of 305 falls off rapidly, however, so it is not suitable for operations that induce severe stretching.
Type 302 is the mid-range choice. Its mechanical properties and forming behavior fall somewhere between that of 301 and 305, so it offers benefits and shortcomings of both.
These martensitic stainless steels and more versatile because they can be strengthened through cold working and heat treatment. Even in the soft-annealed state, 400-series alloys are stronger than carbon steels, elongations are generally lower, and the metals are harder. This more power must be applied to achieve plastic deformation.
When parts don’t sufficiently harden during stamping, the stamper has another option – heat treating. After heat treating, parts are removed from the furnace at relatively high temperatures, from 1750 to 1850°F, and quenched to achieve a specific hardness.
These grades are considered deep-hardening, so quenching in ambient air usually achieves desired results. Water and oil quenching are options in special cases. For 420 and 440A, stress-relieve the parts to avoid a structure that is too brittle. This may vary slightly with sheet metal but the overall takeaways are the same.
If martensitic grades aren’t sufficiently hardenable, a precipitation-hardening stainless steel should be considered. Such steels contain small additions of copper, aluminum, phosphorous or titanium. Parts are cold formed in the relatively soft solution-annealed condition and then age-hardening treated, in which the added elements precipitate as hard intermetallic compounds that significantly increase hardness and strength.
Precipitation-hardening stainless steels such as 17-4PH, 17-7PH, A286 and AM350 are similar and may be used interchangeably, depending on availability in the desired gauge and temper. Due to a significant increase between annealed and final hardness levels, post-heat treatments with these alloys are more involved.
Alloys 17-7PH and A286 can be heat treated in conditions ranging from the annealed or solution-treated condition to a series of cold-reduced tempers that, with proper heat treatment, can produce surprisingly high properties. With multiple heat treatments and tempers to consider, review specific requirements and heat treat cycles with an experienced metallurgist to obtain optimum results for formability and strength.
Both 17-4PH and AM350 are rarely provided in the cold-worked condition due to their high strengths in the annealed condition and the fact that subsequent heat treatments will provide extremely high strength levels. Despite more complex metallurgy, PH alloys are not necessarily more costly than many non-age-hardenable alloys. In fact, performance may be substantially higher in PH alloys without a cost penalty.
Nickel and Nickel Alloys
For this group of alloys, those with higher nickel contents will be easiest to form. These include pure nickel and, among the proprietary alloys, Monel 400 and Inconel 718 and 800. All other nickel-based alloys commonly are cold formed, including Inconel 625, Hastelloy C-276, Hastelloy X and Haynes 230. While none are highly ductile, they can be rolled and annealed to provide acceptable formability. Those that can be effectively cold formed to a final hardness include Inconel 625, Inconel 718, Incoloy 800 and Monel 400. Others are age-hardenable, notably Inconel 718, Waspaloy 5, Inconel X750 and Monel K-500.
Inconel 718 can be cold worked to a desired hardness or age hardened to even higher levels. Using standard cold-working procedures, annealed 718 can be heat treated to levels higher than 180,000 psi while cold-worked material can be raised to minimum levels of 250,000 psi after heat treatment. Consult with an experienced metallurgist for the optimum formability and strength parameters.
The material selected and the processes used to prepare these alloys for stamping must take into consideration some additional factors.
Punch tests are not good hardness indicators on comparatively soft materials, so a material’s grain size often is used to indicate formability. In forming, it is desirable to have consistent grain size. Grain size can be controlled by a reroll mill within a very close range by monitoring the temperature of the annealing furnace and the speed at which the strip passes through the line.
If grains are too coarse or lack uniformity, sidewalls of deep-drawn components may roughen up and “orange-peel.” If the grains are too fine, the material may become too difficult to form. ASTM grain-size scales assign a value of 00 to the coarsest-grained and the softest materials and 13 the finest-grain sizes from a coarse size of 6 to a fine size of 12.
Generally speaking, deep drawing is best accomplished in the grain-size range of 6 to 10 and blanking in the 9 to 12 range. However, formability and strength requirements may dictate a more specific range, provided properties can be agreed upon by the producer of the material. A great deal depends on the depth and complexity of the part, and the number of stations in the progression.
This refers to the tendency of strip to exhibit different properties in the direction it was rolled compared to the opposite direction. Rolling is performed in one direction only, so the more rolling passes occurring, the more directionality occurring.
This is a function of a material’s yield strength. The higher the yield strength, the greater the springback tendency. Depending on the accepted springback level, the tooling designer should work toward a yield-strength range that avoids or compensates for this tendency while designing dies. A designer can estimate how much the part will spring back and then design to overbend it by the same amount. How much, exactly, is difficult to determine, because springback tendency varies from material to material and temper to temper.
If designers are uncertain about what alloy to specify for a high-strength part, they should consult a metallurgist. Once these specialists know how to deep a draw or how severe a bend is anticipated, they can use the material tensile and yield strength data to estimate how much it can be formed without causing a fracture. Sometimes its necessary to overdesign the part in terms of alloy selection just to obtain the needed formability characteristics without risking defects or potential failure sites.
Poor formability has many negative results. Troublesome parts and die-progression samples are in constant evaluation in our department at Ulbrich Stainless Steels to determine where a problem might lie. We look at three prime sources – the alloy, the tooling and the forming operation.
Whether the shortcoming occurs in the alloy or not, this is where adjustments usually are made because it is the fastest, easiest and least costly factor to control. Fortunately, these remedies usually are successful. When they aren’t, the metal former must examine other facets of its operation.