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.
Here are 8 reasons why you might want to consider stress relieving the steel before machining your parts.
High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
Long parts. Stresses tend to display and their effects increase longitudinally.
Assymetric parts– and parts with large differences in section or mass.
To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
To decrease mechanical properties. At higher stress relieving temperatures, hardness, tensile strength and yield strength are reduced while % elongation and 5 reduction of area are increased.
To reduce distortion off the machine. Usually stress relieving is used as a last ditch effort to reduce the distortion that presents after machining a part with some or many of the characteristics given above.
Stress relieving is a lower than the material’s critical point thermal treatment also known as strain drawing, strain tempering, strain annealling, strain relieving, or pre-aging. It is performed to modify the the magnitude and distribution of of residual forces within a cold drawn steel bar, as well as to modify the mechanical properties.
Machinability of carbon and alloy steels is a shear process. Working the metal (Shearing to create chip) provides heat. The subsequent sliding of the produced chip on the face of the cutting tool provides heat as well.
Three ways to improve machinability include
Optimizing the chemistry to provide for a minimum shear strength
Adding internally contained lubricants
Adjusting cold work
The steels that we are talking about are in large part composed of the ferrite phase. This is advantageous to us as machinists, because it has a relatively low shear strength.
Because ferrite is also ductile, it does not cut cleanly and tends to tear. Grade 1008 or 1010 are prime examples of how pure ferrite machines. Long stringy, unbroken chips, torn surface finishes and lots of machine down time to clear “birds nests” are typical results.
Adding carbon up to a point improves machinability by adding a second harder phase (pearlite) into the ferrite. The good news is that up to a point, the chip formation is greatly improved, and surface finish improves somewhat. The bad news is that the shear strength of the steel is also increased. This requires more work to be done by the machine tool.
Addition of Nitrogen and Phosphorous can not only increase the shear strength of the ferrite, but also reduce the ductility (embrittle it).This ferrite embrittlement promotes the formation of short chips, very smooth surface finishes, and the ability to hold high dimensional accuracy on the part being produced. The downside is that these additions can make the parts prone to cracking if subsequebnt cold work operations are performed.
The graph below shows how cold work (cold drawing reduction) works in combination to reduce chip toughness, resulting in controlled chip length, improved surface finish, and improved dimensional accuracy of the part. To read the graphs, the Nitrogen content is shown in one of two ranges, and Phosphorous content is varied as is the amount (%) cold work. You can see how the synergistic effects of these two chemical elements when appropriately augmented by cold work, can drop the materials toughness by as much as 80-90%.
Add to that internal lubrication by a separate manganese sulfide phase or a lead addition, and now you can see how these factors can make grade 1215 or 12L14 machinable at speeds far, far, faster than their carbon equivalent 1008-1010. With greater uptime and tool life.
Internal Lubricant- Manganese Sulfides
And you thought that cold drawing just made the bar surface prettier and held closer in size…
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.
While Austenitic Grain Size is a result of chemistry (composition), the changes that it evokes in our process are a result of material structure and properties, not just the chemical ‘ingredients.’
Steel that is fully deoxidized and grain refined is more sound, less susceptible to cracking and distorting, and more easily controlled in heat treat. Well worth it in final performance compared to the machinist’s increased tooling costs. Here are 5 Ways Austenitic Fine Grained steels can affect your shop:
Poorer Machinability than Coarse Grained Steels. (The hard oxides and nitrides resulting from deoxidation and grain refinement abrade the edge of tools and coatings- this is one reason that you go through more tooling on Fine Grained Steels.)
Poorer Plastic Forming than Coarse Grained Steels.
Less Distortion in Heat Treating than Coarse Grained Steels
Higher Ductility at the same hardness than Coarse Grained Steels
Shallower Hardenability than Coarse Grained Steels.
Fine Austenitic Grain Size is a result of DELIBERATELY ADDDING grain refining elements to a heat of steel. Because these grain refining elements have been added, the steel has a “Fine Austenitic Grain Size.”
In order to make steels with this Austenitic Fine Grained Structure, the steel is first deoxidized , (usually with Silicon) and then Aluminum, or Vanadium or Niobium are added. Aluminum, Vanadium, and Niobium are called grain refiners.
After the Silicon has scavenged most of the Oxygen out of the molten steel, the grain refiner is added. (In this post I’ll stick with Aluminum as the example.) The added Aluminum reacts with Nitrogen in the molten steel to form Aluminum Nitride particles. These tiny particles precipitate along the boundaries of the Austenite as well as with in the Austenite grains. This restricts the growth of the grains.
Because the deoxidation and grain refinement create hard abrasive oxide and nitride particles, they machine and process differently than coarse grained steels.
Fine Austenitic Grain Size appears on the material test report as an ASTM value of 5 or greater. Values of 5, 6, 7, 8, or “5 and finer” indicate that the material is Austenitic Fine Grained. Typically 7 or 8 was reported for the Aluminum Fine Grain steels that I certified.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods for determining Average Grain Size.
To get the Coarse Austenitic Grain Size Story, see our post here.