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: Mechanical properties
Because the steel grain structure is cold forged, rather than cut, rolled threads are up to 30% stronger…
While the strength of a thread is a function of section thickness, a thread rolled rather than cut thread is usually superior in mechanical properties, all other things being equal- here are 6 reasons why:
- The flow of the material by cold work during rolling reinforces the shape.
- The cold working strain increases mechanical properties, Surface Hardness,Tensile Strength, Yield Strength, and the Yield Strength / Tensile Strength ratio.
- The surface finish of the thread flanks is usually smoother due to the burnishing action of the rolls.Smoother finish means better fatigue life and fewer opportunities for stress risers.
- There is more material (section thickness). This results in material savings*, since the diameter of the blank will be between the major and minor diameter of the thread, rather than greater than the major diameter for a cut thread.
- The compressive stresses on the threads resulting from rolling improves the fatigue life.
- The root of the thread has a smoother radius, improving fatigue life.
Pictures courtesy PMPA member Ray Industries
Click here for a video of the thread rolling process in action from PMPA member Rolled Threads Unlimited LLC.
*I got my first “learnin” on rolled threads at my customer Keystone Threaded Products back when my hair was not silver and B.K. (Before Kids): “Why do you always buy funny sizes Jim?” I asked. That was a great first lesson on how engineering can add savings:
As machinists, we seldom encounter microalloy steels. but what do we need to know?
- Microalloy steel is manufactured like any other, but the chemical ingredients added at the initial melt of the steel to make it a microalloy include elements like Vanadium, Columbium (sorry, Niobium for us IUPAC purists), Titanium, and higher amounts of Manganese and perhaps Molybdenum or Nickel.
- Vanadium, Columbium Niobium, and Titanium are also grain refiners and aggressive Oxygen scavengers, so these steels tend to also have a very fine austenitic grain size.
- In forgings, microalloy steels are able to develop higher mechanical properties (yield strengths greater than say 60,000 psi) and higher toughness as forged by just cooling in air or with a light mist water spray.
- Normal alloy steels require a full austenitize, quench and temper heat treatment to develop properties greater than as rolled or cold worked.
Since microalloyed steels are able to get higher properties using forging process heat- rather than an additional heating quenching tempering cycle- they can be less expensive to process to get improved mechanical properties.
The developed microstructure ultimately makes the difference. The microstructure developed in the steel depends on the grade and type.
- Normal alloy steels require a transformation to martensite that is then tempered in order to achieve higher properties.
- Microalloy steel precipitates out various nitirides or carbides and may result in either a very fine ferrite- pearlite microstructure or may transform to bainite.
For machinists, if the steel is already at its hardest condition, the microalloyed microstructure of either ferrite pearlite or bainite is less abrasive than that of a fully quench and tempered alloy steel.
P.S. The non- martensitic structures also have higher toughness.
We don’t tend to machine prehardened steels in the precision machining industry, but if you ever are part of a team developing a process path for machining forgings, or finish cuts after induction hardening, these facts might be good to know.
Martensite.
Georges Basement Bainite 1000X
The role of Manganese in steel in our precision machining shops.
Carbon is a chemical element that is the primary hardening constituent in steel. Manganese is a chemical element that is present in all commercial steels, and contributes substantially to a steel’s strength and hardness, but to a lesser extent than does carbon.
- The effectiveness of Manganese in increasing mechanical properties depends on and is proportional to the carbon content of the steel.
- Manganese also plays an important role in decreasing the critical cooling rate during hardening. This means that manganese helps to increase the steel’s hardenability. It’s effect on hardenability is greater than that of any of the other commonly used alloying elements.
- Manganese is also an active deoxidizer, and is less likely to segregate than other elements.
- Manganese improves machinability, by combining with sulfur to form an soft inclusion in the steel that promotes a steady built up edge and a place for the chip to break.
- Manganese improves yield at the steel mill by combining with the sulfur in the steel, minimizing the formation of iron pyrite (iron sulfide) which can cause the steel to crack and tear during high temperature rolling.
Manganese is an important constituent of today’s steels.
Now you know a few reasons why Mn (the abbreviation for Manganese) is the second element shown on the chemical analysis report (right after carbon).
It’s That Important!
Mn Ore Photocredit.
- Nitrogen strengthens ferrite.
- Nitrogen improves surface finish.
- Nitrogen improves production rates.
- Nitrogen can contribute to cracking during cold working.
Well 3 out of 4 ain’t bad.
Nitrogen is a chemical element that can contribute to improved surface finish, especially on side working tools. It does so by strengthening the chip, resulting in a crisp separation from the workpiece. The bulk hardness of the material increases with increased Nitrogen as well.
Nitrogen is an important factor, especially in free machining steels. Like 1215 and 12L14.
Nitrogen is higher in electric furnace melted steels than in steels produced in Basic Oxygen Furnaces.
The down side of higher Nitrogen is that it can result in cracking during cold work- operations such as staking, swaging or crimping.
Nitrogen is “implicitly” specified whenever purchasing chooses a steel supplier. That supplier’s melt process is a major factor on determining the Nitrogen content that you get in the shop.
For a more complete discussion of the role of Nitrogen and how it can affect your precision machining operations, see our article in Production Machining here.
Photocredit.