Deep-drawing stainless steel and nickel alloys means balancing many opposing factors. Adjusting hardness values to suit process requirements aids in the decision-making process. The defining characteristic of deep drawing, with regard to forming, is flow. The material should flow into the die as it is drawn by the punch. The characteristics of the alloy should be tuned to facilitate the material’s ability to flow uniformly.
By Keith Grayeb, Senior Process Metallurgist & Sean Ketchum, Director of Metallurgy, Ulbrich Technical Services.
The ideal material
Stampers, understandably, want ductile materials that are easily cold-formed, but specifiers of high-performance alloys usually do not consider formability. They only care that the formed part meets the print requirements and has suitable strength for the end-use. The ideal is a material that is ductile enough to draw without rupturing, yet strong enough for the final application. That is a tall order, but with a little help from two under-appreciated factors – work hardening and heat treatment – stainless steel can approach that ideal.
Formability
Forming requires plastic deformation, which occurs somewhere between the yield strength and tensile strength of the material. If the yield is not exceeded, deformation occurs, and there is no net change in dimensions – therefore, no forming. However, every material has limited ductility; exceeding the tensile strength will result in a material fracture. In higher-strength materials, the window between yield and tensile is tiny. It is almost impossible to achieve the desired formability and required tensile strength in the same material without taking some extra steps.
In the case of deep drawing, the material must work-harden fast enough to withstand forming pressures as the part wall thins, yet slow enough to retain the ductility needed for the material to flow into the die. Selecting a proper alloy and starting temper condition is essential in maintaining this delicate balance during the forming process. If the starting material is not strong enough or work-hardens too slowly, it may suffer a ductile tear, as it is stretched too thin. If the starting material is too strong or work-hardens too quickly, it may suffer a brittle fracture, as it bends and stretches beyond its tensile limit instead of flowing. All these forming considerations must be taken on top of meeting the print requirements of the final part.
Usually, repeated press action as a part travels through a progressive die will induce enough cold working to bring the material to a sufficient strength level, around quarter-hard or half-hard. When a given progression does not achieve the desired hardness, stampers have a couple of options. They can beef up tooling and choose a large enough press to cold form a harder and stronger material. Besides its high cost, such an option may result in part tearing or fracture, as well as wear and tear on the tooling and press.
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; however, this adds a secondary operation after the initial forming, which can alter the surface condition and the dimensions of the formed part.
With high-performance alloys, hardness is not the only consideration. The buyer may also want corrosion resistance, high-temperature properties, or 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.
Quarter-hard, half-hard, full-hard, and spring-temper-hard (also referred to as extra-full-hard) are achieved by temper rolling: cold rolling annealed material a specific ratio of starting and final thickness, sometimes called a percent reduction. Hardness will scale with percent reduction based on the work hardening rate of the alloy, which varies with chemistry. 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. There is 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. Uniaxial tensile testing, performed on representative sample coupons extracted from the coil at the final thickness, is necessary for determining tensile strength, yield strength, and percent elongation. Hardness testing is relatively non-destructive and much faster than tensile-testing, so specifying a hardness requirement can simplify testing and allow for easier correlation between the starting and end conditions.
300-series stainless
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 can handle much pulling and stretching, but work-hardens too 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% of stainless deep-drawn parts are produced from this grade. Due to its relatively high nickel content, the material work hardens very slowly during the forming process.
400-series stainless
400 series alloys come in two main varieties: ferritic and martensitic. Ferritic alloys like 430 are magnetic in the annealed condition and can be polished to a mirror-like finish. Martensitic stainless steels are more versatile because they can be strengthened through cold working and heat treatment. Even in the soft-annealed state, 400-series alloys are stronger and harder than carbon steels. As such, more pressure must be applied to achieve plastic deformation. However, these alloys are not as corrosion resistant as their 300 series counterparts.
Precipitation-hardening alloys
If a heat treat (hardenable) alloy with better corrosion resistance or a higher service temperature than martensitic is desired, then 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, then age-hardening treated. This is when the added elements precipitate as hard intermetallic compounds that significantly increase hardness and strength.
Alloys 17-7PH and A286 can be heat treated from the annealed condition, but only achieve their highest strength when heat treated after being temper rolled. 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. Deep drawing either of these alloys is challenging due to their extremely limited ductility in the annealed condition.
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 600, 718, and 800. All other nickel-based alloys are commonly cold-formed, including Inconel 625, Hastelloy C-276, Hastelloy X, and Haynes 230. While none of these alloys are as ductile as 305, they can be rolled and annealed to provide good 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, Inconel X750, and Monel K-500.
Other factors
The material selected, and the processes used to prepare these alloys for stamping, must take into consideration some additional factors:
Grain size
– Generally speaking, the grain size will be an essential value to consider when determining what material is needed for your application. 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 often desirable to have a uniform and equiaxed grains. Grain size can be controlled by a rerolling mill within a very close range through the employment of continuous annealing, where the entire length of the strip soaks at the desired temperature for the same amount of time. If the grains are too coarse or lack uniformity, the sidewalls of deep-drawn components may suffer from a defect called ‘orange-peel’, where small tears form on their surface. 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-grains and 13 to the finest-grains.
Directionality
– This refers to the tendency of the alloy 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. Directional or isotropic conditions can result in a drawing defect called ‘earing’ where the top of the drawn part becomes wavy instead of a uniform circle.
Springback
– This is a function of a material’s yield strength and elastic modulus. The more strain it takes to exceed the yield point, the more a part will spring-back after the forming pressure is removed. Depending on the accepted spring-back 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. If spring back is a concern, tensile properties should be specified instead of hardness to allow for a complete correlation between mechanical properties and forming behavior.
Poor formability has many negative results. Troublesome parts and die-progression samples are in constant evaluation to determine where a problem might lie. Whether the shortcoming occurs in the alloy or not, this is where adjustments are usually made because it is the fastest, easiest, and least costly factor to control. Fortunately, these remedies are usually successful. When they are not, the metal former must examine other facets of its operation.
About the authors
Keith Grayeb acts as Senior Process Metallurgist, and Sean Ketchum is the Director of Metallurgy at Ulbrich Stainless Steels & Special Metals, Inc. Learn more at
www.ulbrich.com.
