Metal strip for stamped parts usually is supplied by service centers that buy the material from large mills and slit it to width for their customers. At least, that is the common perception. But there is another layer in the supply network, which often is unrecognized.
These are the precision rerollers that create the dimensional and metallurgical characteristics that are beyond the equipment capabilities and narrow size range carried by most service centers. Rerollers are growing in importance every year as companies look for ways to increase production, improve cost efficiency and, at the same time, produce a higher level of quality for their customers.
Rerollers exist throughout the metal industry, but in the stainless, nickel and cobalt alloy market they take on greater importance because these higher priced metals are expected to meet more stringent specifications. Minute changes in metallurgy or surface finish can be a key factor in the performance of many stamped parts.
Some typical services performed by reroll mills include: Rolling to precise gauge, increasing formability, adjusting grain size, controlling springback, adjusting temper, improving surface finish and solving customer problems.
Rolling to Precise Gauge
Stainless steel strip and wire, as well as nickel and cobalt-based alloys in various forms are available from integrated producers and made to standard mill tolerances. Rerollers often inventory large quantities of these materials. At Ulbrich, we work with over 165 different alloys in all. These mill partners supply metals which are prepared for stamping by rolling to very close thickness tolerances, customizing the mechanical properties and providing the desired surface finish.
Ulbrich operates multiple rolling mills of various size across our global footprint to handle the many different material requirements they are expected to meet. This equipment enables the company to consistently produce high quality strip to the tightest possible gauge tolerances and maintain a precise, uniform thickness level throughout the entire coil.
If rolling requires several passes, an element of work hardening becomes a factor. This can be removed by processing the material through an annealing cycle in order to return it to the desired level of formability. At Ulbrich, seven controlled-atmosphere, bright annealing furnaces are used to meet these metallurgical adjustments.
In addition to our precision rerolling capabilities, Ulbrich operates several service centers throughout North America which provide supply chain management services for customers who require them. Regardless of your material needs or challenges, Ulbrich is here to serve our incredible manufacturing customers. If you’d like to learn more, give us a call or click here to contact a specialist today!
Naturally, stampers want material that is easy to cold form for their manufacturing operations. Formability, however, is inversely related to the very thing users may want when they specify the more specialized alloys – greater strength and hardness. One example is corrosion resistant springs.
Working an effective trade-off between strength, hardness and formability is one of the major services provided by the metallurgical staffs at the reroll source. With close engineer-to-engineer collaboration with our expert metallurgists, alloys can be processed so they can be drawn, formed, blanked and even deep drawn, just as effectively as other metals. It is important to take into account the tool or machine being used with metal stamping and a re-roller can help make sure the appropriate material is selected for a specific manufacturer.
Another major concern of stampers is springback. This characteristic also can be controlled by annealing the material at specific points in the rolling sequence. However, springback is a function of the relationship between tensile and yield strength of the metal and will vary among alloys.
The metallurgical yardstick most stampers use to gauge the formability of strip products is grain size. If the grains are too coarse, or lack uniformity, sidewalls of deep drawn components may roughen up and show evidence of orange peel. If grains are too fine, the part may tear during forming.
When the grain size of the metal isn’t on target, rerollers can adjust it in their annealing equipment. By closely controlling the temperature of the furnace and the line speed of the strip passing through, it is possible to provide a precise grain size range as measured by an industry wide accepted ASTM grain size value.
Grain size values range from 0 to 13, with the lower end indicating coarser grains and the upper end a finer grain. For most deep drawing applications in stainless steel, 8 to 9 on the ASTM scale is the recommended window.
For blanking, an even finer grain size of 10 to 11 is appropriate. The greater hardness assures a good, clean punch through the material.
If formability were the only objective in producing strip products, meeting the specification would be relatively easy. But there are other factors that must be considered. One of these is rolling the material to specific tempers.
Stainless steel, nickel and cobalt alloys can be specified in either annealed (soft) or in cold-rolled tempers. The tempers are designated quarter hard, half hard, full hard and extra full hard, depending on strength requirements. These tempers are produced by rolling specific percentage reductions in the thickness of the strip.
For stamping applications, most material is supplied in the annealed or soft state. Tempers of quarter hard require significantly more effort to form. Tempers above that level are rarely used by the stamping industry, except for blanking.
Stainless, nickel-based and cobalt-based alloys can be surface finished by reroll shops to a bright reflective finish, a matte surface that carries lubricant more effectively or other special surfaces needed for good welding, brazing or bonding.
Finish can be specified either as an RMS (root mean square) value, or as an arithmetic average of the surface roughness as measured by a profilometer. It is advisable for stampers to indicate surface roughness within a range of acceptable finishes.
Specifying a 10 microinch RMS maximum will result in a bright reflective finish as low as 4. As the roughness of the surface increases, rerollers must operate in a broader range. For a 25 microinch RMS, for example, a designer should specify a 20 to 40 microinch RMS.
Surface finishes can be imparted by the face of the work rolls in the rolling mill. Highly polished carbide rolls create a mirror bright reflective finish. Shot blasted steel rolls are used for 20 to 40 microinch RMS matte finishes. An RMS 60 microinch is about as rough a surface that can be created in the process.
Ulbrich also processes over 70 stainless steel, nickel alloy and cobalt alloys in the metal wire format. In our manufacturing, not only do we provide needed dimensional and metallurgical characteristics for forming parts to specification, but our shaped wire division can also produce special profiles, in addition to round (for cotter pin applications), square, keystone and a number of others.
With all the different alloys in the market now, it can be difficult to find something that meets your exact specifications, especially if you have a stringent aerospace or medical application. The variety of different stainless, nickel-based and cobalt-based alloys now being offered by the integrated mills, the presence of rerollers to tailor them to meet stringent specifications and the effective channel of distribution from the service center industry gives stampers plenty of options in meeting their customers’ requirements. By working closely with all three levels of suppliers, it is possible for stampers to use this marketing arrangement for maximum potential in serving their customers. Basically, re-rollers can work with metal stampers to make sure the chain of supply is there to meet their demand and requirements.