Plate and Sheet Flatness

We would prefer if all plate and sheet were “flat”.  Unfortunately, the reality is quite different and the standard commercial specifications to which these products are produced recognize this reality.  The requirements on the flatness of the plate and sheet products sold by Rolled Alloys vary considerably by the type of product, by alloy, by thickness and by the size of the piece.

AMS and ASTM standards exist which cover the flatness requirements for most plate and sheet items as produced by the mill.  Specifications A 480 and AMS 2242 cover many of the stainless steel flat products.  It is important to remember that these tolerances apply to the MILL produced product, where they have specialized equipment to perform flattening operations.

Flatness tolerances for sheet are published only for “full size” sheets.  Since Rolled Alloys also levels this product, we work to the mill tolerances.  In general, sheets 60” wide and under are to be no more than ½” out of flat when laid on a flat surface.

Plates can be out of flat for a number of reasons.  First, when plates are hot rolled, they are subject to uneven cooling.  This can result in variations in thickness, and either due to this or temperature gradients, will have uneven residual stresses.  As the plates cool, they will distort from the flat condition they appeared to have while being rolled.  Annealing of the plates will add further distortion. The flattening equipment restores the flatness of the plate, within limits, by adding more residual stresses.

Second, since the finished mill plate is only “flat” because of the residual stresses imposed to hold it in place, anything that relieves those stresses will lead to a plate that is less flat.  Cutting the plate into smaller sections, exposing it to heat (plasma cutting or welding) or machining away surfaces all result in a relief of stresses that can affect flatness.  For this reason, plates cut from mill master plates are not subject to the same tolerances on flatness.  In fact, there are technically no tolerances for such products but we generally attempt to hold the mill tolerances for cut pieces.

The flatness requirements in ASTM A 480, for plates produced to A 240, were simplified recently to a 36” rule.  The method of measurement is to place the 36” rule on the concave side so both ends of the rule are in contact with the plate surface.  The gap thus created is not to exceed the amount shown in Table A2.20, recreated below in abbreviated form.

 

Thickness from

To Under Thickness

Tolerance

3/16”

<1/4”

7/16”

1/4"

3/8”

3/8”

3/8”

1”

5/16”

1”

10”

1/4"

Two additional notes on this are that if the longer dimension is under 36”, then the tolerance is ¼” and if the minimum specified yield strength of the material exceeds 35 KSI, the values are multiplied by 1-½.  This yield strength caveat would apply to materials such as RA253MA®, AL-6XN® Alloy, RA2205 and RA17-4 PH.  The AMS tables on flatness are different, depend on length and width, but result in similar allowances.

36" Rule - Must be equal to or less than the flatness tolerance

The flatness issues for nickel alloy products are even more convoluted.  Part of the reason for this is that, 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 product when so ordered, 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 the stainless steels.  It is important to remember that many of the nickel alloys are also higher strength materials with a minimum specified yield strength that exceeds 35 KSI.

From an end user point of view there are several things that 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 originally supplied by the mill.  In fact, since cutting relieves stresses it is likely to be slightly 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 needs to 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 working to improve flatness should be done at room temperature.  For stainless and nickel alloys the actual reduction in strength at temperatures up to about 800° F is minimal.  Most materials should avoid temperatures in the range above 800° F to their annealing temperatures, since they will likely suffer from reduced ductility, not improved.

Second, since the availability of true flat or square bar is declining, more of the requirements for these types of products are being replaced by stripping plate products.  While there is little or no difference between true flat bar and plate materials in terms of mechanical and heat or corrosion resistant properties, there are some physical differences between these two products.  Size tolerances on true bar products are smaller than for plate products.  This is primarily 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 being used more frequently as machining stock where it is replacing forgings or castings that required a greater removal of stock.  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 for complete removal of the hot rolled surface to an imperfection free 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 in 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.

The commercial standards for plate and sheet products are available at a nominal cost from ASTM International (www.astm.org) or SAE International (www.sae.org).  These not-for-profit organizations make these standards available in a number of ways including by facsimile and via the internet.  You may also contact our Quality and Technical personnel for information specific to your needs.