Three primary criteria for selecting bar steels are 1) suitability for end use, 2) suitability for manufacturing process, 3) economical delivery of the requirements.
Suitability for end use includes appropriate mechanical properties, physical properties and chemical compatibility. Mechanical properties can include hardness, tensile and yield strength, ductility as measured by % elongation or % reduction in area, and / or impact properties. Mechanical properties can be achieved by chemical composition, cold work, or heat treatment. Note: properties need to match the environmental conditions of the intended end use… Physical properties that are often considered include magnetic properties for solenoid, actuator, or electronic applications. Process path of steelmaking can play an important role in determining these properties.
Suitability for manufacturing requires at least a cursory understanding of the intended process path. Will there be extensive stock removal by machining? Welding, brazing or other means of bonding? Heat treatment? Will the equipment used to machine require tight dimensional tolerances or straightness? Will the material be upset or cold worked? Will the material be cold worked (crimped, swaged, planished or staked) after machining? Bismuth additives can prevent achievement of bond strength in brazed joints unless special techniques and materials are employed. Various chemical constituents can have an effect on the cold work response of steel. These too can be determined by the melting and thermomechinical history of the steel before it arrives at your shop.
Economical delivery of requirements means choosing a materal that permits the creation of conforming parts that fully meet the requirements for end use and manufacturability at a total lowest cost. There are many ways to meet any particular set of requirements for steel in most uses. Chemistry, cold work, heat treatment, as well as design details can all be criteria used to select one material over another. Minimizing costs is clearly important, but most important is assuring that all of the “must have” properties (strength, hardness, surface finish, typically) needed in the finished product are delivered.
Costs of manufacturing can make up a large fraction of the final products cost. For some parts, the cost of manufacturing and processing can exceed the cost of the material. Choosing the lowest cost process path that will assure required properties often requires steel materials that are priced above the cheapest available. This is because free machining additives, or cold finishing processes can reduce cost to obtain desired properties or product attributes when compared to those needed to get hot rolled product up to the desired levels of performance.
Bottom line: Buyers may want to get the cheapest price per pound of steel purchased; Savvy buyers want to buy the steel that results in the lowest cost per finished part- assuring that costs are minimized for the total cost of production of their product. Understanding the role of steel making and finishing processes can help the buyer optimize their material selection process.
Photo courtesy of PMPA Member Corey Steel.
Tag: Straightness
Straightness is perishable in bars. Straightness is often lost during handling operations, loading and unloading.
Straightness is critical for holding position and tolerances on today’s highly engineered medical, aerospace, automotive and electronic parts.
Here are six ways that bars can lose their straightness.
Six Ways Bar Straightness Can Be Degraded
1) Mechanical damage to an end. If the bundle is struck by a lift truck, or if the bars catch on a rack or table while being hoisted with the crane, this can cause the ends of the bars that are caught to be deflected out of the bundle and bent.
2) Improper blocking and support at mill or on truck. Cold finished barstock, especially smaller diameters, really needs to be supported at multiple points along its length. This reduces the possible radius that the bundle can sag or droop between supports. The mills that I’m familiar with, (PMPA Tech members) are pro’s and know the best way to support the product and to package it securely. Reputable mills put more bands on smaller ID bar bundles to preserve straightness when needed.
3) Truck loading and securing. The binders used to secure the bars onto the truck can cause a permanent deformation if they are not matched up with the blocking beneath the bars. I saw a trucker once use a 4 foot piece of pipe as a “cheater” to secure the binding chains ‘one more notch.’ You could hear the wood underneath the bundle being crushed. Chains are always bad news for cold drawn bars- nicks, and gouges and ‘low spots.’
4) Improper unloading. Putting a spreader bar on so that there are multiple points of support for the bundle is critical, especially with the smaller diameter bars and small bundles.
I have seen shops unload bundles by using a single “hitch” at the approximate middle of the bundles. This causes a permanent camber over the length of the bars. Jerky crane lifts rather than smooth movements can magnify this effect.
5) Hand unloading or using a forklift. Small diameter bars especially can be bent by the way they are manually pulled out, lifted, and carried, instead of being placed on a table or rack. Using a forklift can also cause bars to be bent.
6) Frequency of handling. If you are buying from a service center, the number of times that the material has been handled can double or triple, compared to a direct shipment from the mill. Given that you may be buying non-bundle quantities, it is a fact of life that the number of lifts and handling increases dramatically with the additional destination of the service center, as well as in the act of splitting the bundle.
When you encounter bar straightness issues, characterizing the way the bar deviates from straightness can help you determine which of the above factors might be the cause.
