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: Percent Elongation
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.
Ductility is often indicated by chip control issues in certain steels, as the chip readily deforms but does not separate from the work piece. This can result in persistent burrs attached to the work .
Ductility can also mean long stringy chips that can form a dreaded “birds nest” engulfing the tool and work piece.
Long necklace chips are another sign of ductile materials in machining.
Short chips curled into “sixes and nines” showing a bit of heat discoloration are typical of less ductile materials and dutile materials machined at proper parameters using chip breakers and high pressure coolant delivery.
In our machining practice we would prefer materials that are “crisp” rather than ductile.
In order to successfully deal with ductile materials, strategies such as chip control features on inserts, wiper style inserts, through tool coolant, interrupted cuts, chip breakers, and high pressure coolant can be considered.
Dialing in the appropriate feeds, speeds and depth of cut are crucial too.
Birdsnest photo courtesy Garage Journal
All other photos by author.
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.