The ability of a material to deform plastically without fracturing, is called ductility. In the materials usually machined in our shops, ductility is measured by determining the percent of elongation and the percent reduction of area on a specimen during a tensile test.
Our earlier post about Ductility showed how ductility can impact our shops. In this post, we will describe how we can measure ductility and use it to predict behavior based on values reported on certs and test reports.
The percent elongation and percent reduction of area values shown on our test reports and material certifications from our material suppliers indicate the ductility of the material tested.
In the tensile test, a cylindrical specimen is gripped securely and subjected to a uniaxial load and elongated until it breaks. At the end of the test, the pieces of the fractured specimen are fitted back together again and the change of length between the two gage marks put on the specimen before testing is determined. The change is then expressed as a percentage of the original gage length.
The percent reduction of area is determined by measuring the minimum diameter of the broken test specimen after the two pieces are fitted together and the difference is expressed as a percentage of the original cross sectional area prior to the test.
A minimum of 12% elongation is recommended for consistent, trouble free thread rolling applications.
Rolled threads are stronger, so having the ductility to thread roll is important. However, too much ductility makes it difficult to get the chip to separate by cutting.
Low ductility can be problematic for cold deformation manufacturing processes such as thread rolling, cold forming, swaging, staking and crimping.
This is the designer’s compromise: if it is good for cutting, it is probably not very good for rolling.
HSC online Graphic of test specimens
Yost made in USA vise photo credit
Tag: Swaging
Cold work is defined as the plastic deformation of a metal below its recrystallization temperature.
In the precision machining industry, cold working processes can include thread rolling, thread forming, swaging, crimping, staking, planishing, and metal spinning.
And the steel bars that we machine are typically cold drawn (cold worked.)
Our suppliers use cold work when cold drawing a bar from hot roll to make it more machinable.
How to recognize a cold work process: No heat is added and no chip is removed in the process of moving the metal into shape.
Cold working of steel
- changes its mechanical properties
- and improves its surface finish.
Tensile strength and yield strength are increased by the cold work while ductility (as measured by % elongation and % reduction in area decrease.
See our post here.
Steels with low carbon contents, low residuals, low Nitrogen levels, and made by the Basic Oxygen Process readily cold work- think 1008, 1010, etc..
Intentionally adding nitrogen can make predispose a part to cracking during cold work. If a part needs to be crimped, swaged, staked or otherwise cold worked after machining, You should make certain that the steel is not renitrogenized. (Nitrogen intentionally added during the melt process).
Also, make sure that the cold work in cold drawing was standard draft rather than heavy draft. Heavy draft reduces the ductility remaining in the bar- but makes the chips easier to separate.
We posted about these issues here.
More information on Nitrogen in free machining steels.
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.
Keep these 6 Keys to Using Free Machining (12XX) Steels in mind:
- These steels are not generally sold for applications requiring high standards of strength, hardness or other related properties. Applications where vibratory, torsional or alternating stresses approach the grades’ static limits are NOT recommended.
- These steels are frequently case hardened or carburized in order to achieve desired surface hardness.
- When cold drawn, these steels can be notch sensitive. Highly polished fatigue specimens may achieve expected endurance values, but poor surface finish, tool marks, or sharp corners in the design may cause lower than expected performance.
- These grades have relatively low impact strength at reduced temperatures and should not be used for sub-zero impact applications.
- These steels are not recommended for applications where severe cold work follows machining. Crimping, staking and swaging may be performed, especially in non-renitrogenized grades. But severe crimping, cold metal movement, and bending may not be satisfactory in these grades.
- The addition of Lead or Bismuth does not alter the mechanical properties in tension. 12L14 and 1215 of same nominal size and process will be indistinguishable by hardness or tensile testing.
Free Machining Steels in the 12XX series- 12L14, 1215, etc., are selected in order to reduce the time needed to make large volumes of complex parts. This reduces the cost per part. The usual application is one where bulk and shape (mass and geometry) are the chief requirements. The factors that make these steels highly machinable also influence behavior of the products in service. Designers and engineers should keep the above 6 Keys in mind when considering the material for an application.
6Keys: Photo credit .
- 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.