The flatness issues for nickel alloy products are even more convoluted. This is partly because, historically, different mills produced their proprietary alloys to match their capabilities. Although most of these materials are now in the public domain, they are still produced to separate ASTM specifications, and the actual requirements can vary from one alloy to another. AMS tolerances for nickel alloys are uniform and apply to nickel alloy plate products. However, these tolerances are also broad. For example, a 96″ wide, ¼” thick Alloy 625 plate could deviate from flat by as much as 2.44″. Fortunately, these products are typically produced to more reasonable levels of flatness and usually meet the flatness requirements used for stainless steel. It is important to remember that many nickel alloys are also higher-strength materials with a minimum specified yield strength exceeding 35 ksi.
From an end-user point of view, several things should be kept in mind relative to plate flatness. First, when a plate is cut into smaller pieces, it will not be any flatter than what was initially supplied by the mill. Since cutting relieves stress, it is likely to be less flat. For this reason, the flatness tolerances that apply are those that applied to the original mill plate, not the cut size. It is important to be familiar with the flatness allowances permitted in the basic specifications. If these tolerances are too large for the intended application, some consideration must be given to identifying a means to bring them into tolerance for the intended use. This could involve press flattening, hand working, or machining. Any mechanical work to improve flatness should be done at room temperature.
Second, since the availability of true flat or square bars is declining, more of the requirements for these products are being replaced by stripping plate products. While there is little or no difference between true flat bar and plate materials regarding mechanical and heat or corrosion-resistant properties, these two products have some physical differences. Size tolerances on true bar products are smaller than plate products. This is because true bar products are smaller than large mill plates and are produced on equipment with better control over the width and thickness. The camber (edge curvature), flatness, and twist of these smaller sections can be controlled. On flat bar products stripped from plate, the flatness is dependent upon the original plate flatness plus the amount of distortion that occurs due to stress relaxation when they are stripped. Camber and twist can also result from stress relaxation and cannot be controlled. Depending on the width and thickness of the strips being produced, they may be candidates for an optional machine straightening operation. However, this technique is restricted to thicknesses under ½” and widths under 8″.
Finally, plate products are used more frequently as machining stock, replacing forgings or castings that require more significant stock removal. The advantages are lower costs and better availability. In addition, rising raw material costs drive end users toward lighter gauge raw materials. While plate products are a more economical choice for many of these applications, the potential variation in flatness and the clean-up allowance required to remove the hot-rolled surfaces cannot be ignored. An allowance of 0.040″ per side is generally needed to remove the hot-rolled surface. While hot-rolled plate products are usually on the heavy side, they are permitted to run as light as 0.010″ under the nominal thickness. Add a potential out-of-flat condition of as little as 1/8″, and a 1″ plate might not clean up until it is 0.670″ thick.
