Aug
10

Stainless Steel Grade Selection for Strip, Foil & Wire

Why are raw material requirements so important?

The selection of a stainless steel alloy begins with a simple question: “What is going to be done to the material?” This rather gentle inquiry rapidly cascades into an avalanche of significant questions with considerable consequences. What type of corrosive environment will the material be in? Is the final part under a tensile or compressive load? Are there cyclic forces involved? What is the final market? What types of forming operations are required to make the part?

The details describing the thought-processes for material selection for all possible questions for all possible end use markets would take years to read and even longer to write. Thankfully however, much research has already been generated by industries involved in such questions. The answers are usually available through research. These answers might already be documented by the company asking these questions.

With so much information available (visit Ulbrich.com for a full list of products), and so many questions to ask, let us start with one of the first conversations that Ulbrich will have with companies like your own. This conversation is about material selection based on forming operations. Stainless steels and nickel alloys can be bent, stretched, drawn, coined and pierced. Each one of these processes favors certain characteristics of chemistry and mechanical properties. An analogy is that nails can be hammered into wood, concrete, fiberglass, aluminum and vinyl but only the proper nail for each material will work. Using a wood nail in concrete can result in poor performance and even catastrophic failure. Using the wrong alloy type or wrong mechanical property set can result in poor performance and even catastrophic failure.

Discussing the needed forming operations and the end-use requirements allows us to engineer a balance between the best aspects of stainless steel: ductility and strength. Raw material must be ductile enough to form the part, and the part must be strong enough to handle its application. Let us take a look at each type of forming operation and describe what material best fits.

Drawing and Coining

Drawing

Drawing is a process in which a blanked section of material is pushed into a forming die by a punch. Deep drawing is when the depth of the final part is greater than the part diameter. Drawing can be accomplished in one die/punch session or several. The major raw material need for this process is a low work hardening rate. A low work hardening rate is accomplished by higher amounts of nickel in the stainless alloy. Type 305, with nickel content between 10 – 13%, is a perfect fit for drawing. A low work hardening rate means that the yield and tensile strength increase slowly with increased forming.

Grain size must also be considered when purchasing raw material for drawing operations. Coarse grains can cause a condition known as orange-peel to develop during drawing operations. This undesirable condition creates surface aberrations that resemble an orange peel. Coarse grains can also cause poor ductility. Ductility is a benchmark of drawing success. Coarse grain issues means that a finer grain structure is best for drawing operations.

Coining

Coining is another type of stamping that relies on plastic flow. Work hardening rate (flow) plays an important role in coining operations for similar reasons as it did with drawing operations. Coining is easier to accomplish with lower work hardening rates. Grain size is also a consideration. Although a finer grain is a good choice for almost all drawing operations, the grain size required for coining depends on a combination of factors unique to the finish part’s requirements.

Bending, Stretching & Piercing

Bending

Technology is advancing at paces more rapid than ever. Aerospace, computers, and even safety razors are growing more complex every day. These large complexities are comprised of equally complex components. These components can have very strict requirements for dimensions to ensure proper function. This means that manufacturers must be able to bend steel into difficult shapes. A manufacturer must maintain a prescribed degree of strength while simultaneously planning for spring-back. Strength is planned from raw material mechanical properties & thickness, degree of bending and work hardening rate. Spring-back is determined by thickness and final mechanical properties. Clearly, these variables represent a complex engineering challenge. A material with an acceptable work hardening rate must be ordered to the correct raw material mechanical properties. This material must be bent to the proper dimensions, then over-bent to accommodate the material’s inherent spring-back of the current mechanical properties of the bent piece.

So let us dissect this problem by starting with our previous drawing example using type 305 stainless. 305, as we discussed, has a low work hardening rate. This means that a large degree of cold work, even from bending, will not increase mechanical properties much. To a manufacturer, this means that the final component will not be very strong. This is not what we want. So let us take a look at a different material: 301 stainless steel.

301 contains 6 – 8% nickel compared to 305’s 10 – 13%. Nickel is a primary contributor to work hardening. This means that when work hardening 301 and 305 to the same amount, 301 will have work hardened more which results in higher mechanical properties. This phenomenon is highlighted in figure 1. In this example, for the same amount of cold work, the tensile strength of 301 is 80,000 Psi greater than its 305 counterpart. A part manufactured with a bending process will be stronger than an identical part made with 305.

Stretching

Stretching, at times, can appear similar to a drawing operation. However, drawing operations include an ironing component from drawing dies. This ironing helps material to flow rather than stretch. Stretching operations are very dependent on a higher work hardening rate. If we look at figure 1 again, specifically at our prescribed percent of cold work, the 305 sample failed with lesser tensile stress than the 301 sample. The 301 sample increased strength rapidly with higher applied loads. As the load increased, the new higher inherent strength resisted failure. Since stretching does not have an ironing manufacturing component, a material must stretch and strengthen itself by work hardening.

Piercing

Piercing involves using a punch and die to shear raw material. Spring-back is the largest concern in piercing operations. Piercing is easiest to accomplish with materials that have minimal spring-back. Typically, this means lower elongation values rather than higher elongation values. Elongation is a direct reflection of the ductility of a material. High elongation means ductile, low elongation means rigid. Spring-back issues arise when a punch is retracted after piercing a material that is too ductile. The raw material strip can “grab on” to the punch and the material strip can be pulled upward. Rigid material strip does not have the tendency to do this. In some piercing applications, it may be necessary to specify a maximum allowable elongation value.

Summary

If a product only requires one forming operation, such as stamping with only bending, then the decision on an alloy and properties can be based on stretch-ability and spring-back. Parts may involve a combination of bending, stretching, coining, piercing and drawing. These situations can be complex and involve trade-offs for desired properties. In this type of situation, the most difficult operation may dictate what the others have to accommodate. The tensile test is a good indicator of properties. Comparing the average yield, tensile and elongation values of different alloys in different tempers gives insight into the possibilities. The initial meeting between customer and supplier is usually a valuable two-way-review to determine the best options.


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