Technically Speaking
Advances in Springmaking Materials — Part 2: Rod Rolling By C. Richard Gordon (Editor’s note: Rick Gordon’s article was developed as part of the overall theme of “Advances in Springmaking.” This is the second in the series articles on advances in springmaking materials. Additional articles on wire drawing and heat treating are planned for future issues of Springs.)
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dvances in the production of springmaking materials over time have resulted in improvements in spring performance. Since the majority of springs are produced from carbon and alloy steels, production methods of these materials will be described here. Production improvements have occurred in the areas of steelmaking, continuous casting, rod rolling, wire drawing and wire processing (heat treatment). Improvements in springmaking machine technology were discussed previously1. Improvements in the areas of steelmaking and continuous casting were also discussed previously2.
Materials Design Philosophy In the Fall 2019 issue of Springs3, I discussed the relatively simple engineering concept known as the materials design philosophy, which can be used as a tool to better understand compositionproperty relationships. Briefly, the materials design philosophy shows the relationship between the composition of the material and the final product properties (Figure 1). A material with a specific composition is processed in a given operation. The combination of the material and the processing results in a particular structure (metallurgical microstructure). The microstructure in turn results in the specific material properties. In reality, the process of design occurs from right to left. Engineers are interested in specific material properties, which in turn are dependent on the structure, which is a function of designed processing of a material with a specific composition. Material properties such as tensile strength and ductility are determined through mechanical testing, which have been discussed in past Springs articles4,5,6,7,8,9. For the purpose of this article, Figure 2 shows an expansion of Figure 1 which includes our process of interest: rod rolling. From a materials perspective, a goal of any process improvement is to improve the uniformity of the product as well as enhance product properties. From an operations perspective, a goal is to improve productivity with the attendant reduction in cost.
Composition
Processing
Structure
Properties
Figure 1. Materials Design Philosophy
Thus, the challenge is to obtain both the materials goal and the productivity goal; how to achieve the balance is a subject beyond the scope of this article. In this article, only the rod rolling process is discussed.
Rod Rolling In the past, billets used to hot roll wire rods were produced from steel cast into ingots. Ingots were rolled to billets, which were subsequently rolled to rods. Today, essentially all steel is produced using the continuous casting process. For the highest quality high carbon wire products, such as valve spring quality wire, continuously cast blooms are rolled to billets. Billets are then subjected to surface inspection and conditioning prior to rolling to wire rods10. These steps are aimed at producing wire rods with the highest level of surface quality. Over time, there has been a dramatic improvement in hot rolled steel wire rod processing speed due to equipment improvements. This has resulted in new modern single strand wire rod mills replacing older mills with four strands. A top finished rolling speed of 110 mps (21,654 fpm) is routinely achieved. Converting billets into wire rod is an extremely complex process that requires excellence in a multitude of areas to consistently manufacture high quality finished products11. An excellent article is included as reference 11, which describes many of the improvements made in rod mill rolling equipment and operating practices which can be utilized for superior product production. Rick Gordon is the technical director for SMI. He is available to help SMI members and non-members with metallurgical challenges such as fatigue life, corrosion, material and process-related problems. He is also available to help manage and oversee processes related to failure analysis. This includes sourcing reputable testing labs throughout North America, forwarding member requests to the appropriate lab and reporting results and recommendations. He can be reached at c.richard.gordon@gmail. com or 574-514-9367.
SPRINGS / Fall 2021 / 21
