We shall discuss How do alloy characteristics impact formability and deep drawing .
Deep-drawn stainless steel and nickel alloys mean balancing many conflicting factors.
Adjusting hardness values to process needs aids in the decision-making process.
Printed parts go into stamping plants to obtain deep parts made from familiar grades of stainless steel.
This is not a problem until you realize that the intensity distribution of the finished part is slightly higher than that typically produced during this process.
A specification calling for tensile strength of 175,000 psi and yield strength of 135,000 psi becomes a three-quarters hard material.
difficult to form. In stainless steel, the preferred range of input material is between the annealed and semi-hardened conditions.
ideal material It will be appreciated that the stamp requires a flexible material that is easy to cool. But specifiers of high-performance alloys often don't consider ductility—they want strong, hard, tough parts.
The ideal material is one that bends easily during the forming process but does not bend at all once a piece is made.
But with the help of two underappreciated factors (work hardening and heat treatment), stainless steel can approach this ideal state.
plasticity Forming occurs between the yield strength and tensile strength of the material.
If the yield is not excessive, forming will not occur, but excessive tensile strength can cause material failure.
In higher-strength materials, the window between yield strength and tensile strength is very small.
It is nearly impossible to achieve the desired ductility and desired tensile strength in the same material without additional steps.
Often, the repeated pressure of the part as it passes through the progressive die creates enough cooling to make the material a quarter or half as hard, which is usually sufficient.
When this cold working does not produce the desired hardness, there are several options for stamping.
They can strengthen the tool and choose a press large enough to make harder, stronger materials in the cold.
In addition to being costly, this option can lead to tearing or breakage, as well as tool and press wear.
Tensile testing is always done on the material, which is a good indication of how it will perform in deep tensile testing.
A better option might be to shape the part first and then heat treat it to increase hardness and strength.
For this option, the alloy must be selected with two hardness values in mind: sufficient cold forming ductility and sufficient hardness to meet
finished part specifications.
Hardness represents strength. For high-performance alloys, hardness is not the only consideration.
Buyers often want corrosion resistance, high-temperature performance, and other properties.
The job is to find an alloy of that specification that can increase the hardness grade to whatever value the buyer wants.
To minimize costs, this should be done with a minimum number of rolling mills and furnaces in the material supplier and a minimum number of stamping stations in metal forming operations.
Of course, this can sometimes depend on thickness.
difficulty levels Quarter-hard, semi-hard, full-hard, and spring-hard (also called super full-hard) are obtained by turning a specific reduction
percentage on the annealed material. The hardness values listed here are actual specifications, not just rules of thumb - they are contained in
ASTM specifications, which refer to specific levels of tensile strength.
The hard quadrant nominal minimum tensile strength is 125,000 psi.
150,000 psi for half-hard, 175,000 psi for three-quarter-hard, and 185,000 psi for full-hard. You'll find a difference of 25,000 psi between each
graph, except for three quarters and full hard, where the data converges as the work hardening curve flattens.
These hardness values also have a minimum yield value.
On Ulbricht stainless we use 75,000, 110,000, 135,000 and 140,000 psi.
Pulling the sample on a tensile tester is the preferred method for measuring these properties.
But Rockwell hardness testers are more common in stamping plants than tensile testing equipment, so the gauge, Rockwell C value is
particularly useful for 301 and 302 stainless sheets of steel, but not very accurate for alloys with lower work hardening rates.
300 Series Stainless Steel 300 series austenitic stainless sheets of steel are hardened by cold work only - heat treatment is not an option.
Since cold working occurs in the plastic range between yield strength and tensile strength, looking at the properties table may suggest that Type
301 is a good candidate for stamped parts due to its relatively wide range.
This class-leading cab can handle a lot of pull but tends to go fast.
Type 305 has a much narrower range between yield strength and tensile strength, but it is the grade of choice for deep drawing applications.
About 90% of deep-drawn stainless-steel parts are produced from this grade.
Because of its relatively high nickel content, work hardening increases very slowly during forming.
It can go through a series of pulls without becoming too hard or brittle and can often be stretched extensively before needing to be annealed.
However, the good early elongation of 305 decreases rapidly and is therefore not suitable for operations that cause severe stretching.
Model 302 is a mid-range option.
Its mechanical properties and molding behavior are between 301 and 305, so it has advantages and disadvantages of both.
400 series stainless steel These martensitic stainless steels are more versatile because they can be strengthened by cold working and heat treatment.
Even in the soft annealed state, 400 series alloys are stronger, generally less tensile, and have harder metals than carbon steel.
More power must be applied to achieve plastic deformation.
When the part is not sufficiently hardened during the stamping process, there is another option for stamping - heat treatment.
After heat treatment, the parts are removed from the furnace at a relatively high temperature, from 1750 to 1850 degrees Fahrenheit, and then
quenched to a certain hardness. Grade 410 stainless steel is typically hardened able between Rockwell C35 and 45, while grade 420 is hardened
able at low to medium C50 and grade 440A is hardened able at high C50 and low C60.
Precipitation-Hardening Alloys
If martensitic grades are not hard enough, then precipitation hardening stainless steels should be considered.
This steel contains small amounts of copper, aluminum, phosphorus, or titanium.
Parts are cold formed in the relatively soft solution annealed conditions and then age hardened, in which added elements are precipitated 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 can be used interchangeably depending on
the required specification and temper availability.
Post-heat treatment of these alloys is more important due to the significant increase between annealing and final hardness levels.
Alloys 17-7PH and A286 can be heat treated under a variety of conditions, from annealing or solution working to a series of reduced cold
tempers, and with proper heat treatment, surprisingly high performance can be produced.
Consider multiple heat treatments and tempers and review specific requirements and heat treatment cycles with an experienced metallurgist to
achieve the best results for ductility and strength.
17-4PH and AM350 are rarely available in cold work because they have high strength in the annealed condition and subsequent heat treatment
provides very high strength levels.
Although the metallurgy is more complex, PH alloys are not necessarily more expensive than many non-age hardenable alloys. In fact, the
performance of PH alloys may be higher without increasing the cost.
The punch test is not a good indicator of hardness for relatively soft materials, so material grain size is often used to indicate ductility.
During molding, it is desirable to have a constant grain size.
The roller can control the grain size in a very close range by monitoring the temperature of the annealing furnace and the speed of the strip
passing through the line.
If the grains are too coarse or uneven, the sidewalls of deep drawn sections can become rough and "flaky".
If the particles are too small, the material may become too difficult to shape.
The ASTM grain scale assigns a value of 00 for the coarsest and softest materials and a value of 13 for the finest materials, ranging from 6 for the
course to 12 for the finest.
In general, deep drawing works best in the 6 to 10-grain size range and blanking in the 9 to 12 range.
However, ductility and strength requirements may dictate more specific ranges, if material manufacturers can agree on properties.
Much depends on the depth and complexity of the production and the number of stations in progress.
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