To create a vast array of metal covers and boxes, industrial and consumer components, office furniture, and the like, bending machinery is often used to shape metal sheets or plate into various shapes. Two things critical in this process are the capacities of the press brakes used, and the yield strengths of the materials going into them. Simply put, if the bending machinery is not capable of overcoming the materials yield strength, no permanent bend in the part will occur.
Permanent plastic deformation (actually putting a permanent bend in the part) only occurs after enough force has been applied to the material to exceed a given material’s yield strength. This is quite different from beginning to apply pressure to the material and causing it to start to bend (overcoming the materials proportional limit). If one does not exceed the materials yield strength, no permanent bend will occur, and the material will “spring back” into its original condition once pressure has been removed.
Taken to the other extreme, is when too much force is applied and the material is subject to pressures that exceeds its “facture point.” At this point the material ceases to bend and starts to crack or fracture in the bending machinery.
Trial and error is always one method available to determine a particular material’s yield point, but it is certainly not the best method to use. Using ones knowledge of material principles of proportional limit, yield strength, and facture point, one can experiment with the material in press brakes to see “what works” and what doesn’t,” although there are easier, faster, and better ways to go about this. One of the best, and the easiest, is to request material certifications from the material supplier that will detail the yield strength of the material in its “as purchased” condition. If material certifications are not available, some other methods to use are
Send a sample of the material out to a metallurgical testing lab to have them determine the yield strength of the material
Check the materials actual Brinell or Rockwell hardness, and using reference materials, determine the yield strength ranges for this specified condition by using metallurgical charts or other online resources.
Seek advice from your material supplier for ranges of yield strengths that they supply the material in
Three key elements will impact the yield strength for a given material
The carbon content of the material
The alloys contained in the material
The hardness of the material
One should be cautioned that there are a wide range of yield values for different alloyed steels at different levels of heat treatment and varying carbon contents. Even within a specific material designation, there is enough allowable variation in alloy content, carbon content, and finished hardness to generate a range of yield values for materials in their “as furnished” conditions. This fact warrants further investigation before attempting to bend.
A material like ASTM A366 (alloy 1008) as supplied by the mill may have variations according to published standards from 26,100-34,800 psi for its specified yield strength range. One needs to be aware that this broad range itself may significantly impact overall bending machinery performance. In addition, a material like 1144 (stress proof-equivalent) with its yield rating of 100,000 psi is a whole other story when it comes to being able to produce a bend in the material in press brakes.
The level of success that one has with their bending machinery will be determined in many ways by the condition of the materials that they put into it. With a major investment in press brakes, investing a small amount of time to know what the condition of the materials that you are putting into them is warranted.