11 minute read
Technically Speaking
Types of Tests Used to Characterize Springmaking Materials—Part 3: The Torsional Ductility Test
By C. Richard Gordon
In the Winter 2020 issue ofSprings 1 , I discussed the tensile test as an important mechanical test for springmaking materials. In the Spring 2020 issue ofSprings 2 , I discussed the coiling and wrapping tests as important technological tests for springmaking materials. This is the third article in the series on the subject, which will discuss another important test used to characterize springmaking materials, the torsional ductility test, or torsion test. This article includes materials from the presentation that I made at the 2019 SMI Metal Engineering eXpo in Pittsburgh 3 .
Overview
In general, the testing of materials represents an important part of all quality work. It can include the control of incoming raw materials, materials in production, and produced materials or components before delivery.
Many different techniques are used, including chemical analysis; microscopy; nondestructive testing; mechanical tests such as tensile strength, hardness and fatigue; and technological tests such as bending, torsion, coiling, wrap and weldability
In this series of articles, we have focused on mechanical and technological tests used to characterize springmaking materials. In this article, the torsional ductility (torsion) test will be discussed.
The uniformity of tensile strength and ductility of the wires used to produce springs are critical to successful spring production and end use. The torsion test can be good indicator of these properties, but primarily as a measure of ductility.
Torsion Testing
Torsional ductility is an important quality parameter for assessing the suitability of wire for many wire products, including springs, because many products and components are subject to torsional forces during operation. It is included as a specification parameter for many sophisticated product standards worldwide because the test can identify suspect material which may result in a spring failure.
Torsion tests can be performed by applying only a rotational motion, or by applying both axial (tension or compression) and rotational forces to the sample of interest. The type of torsion testing varies from product to product but can be characterized as failure, proof or product opera tion testing. • Failure testing – Twisting the product, component or specimen until failure as an indication of ductility. • Proof testing – Applying a torsional load and holding this torque load for a fixed period of time. • Operational testing – Testing complete assemblies or products to verify that the product performs as expected under torsional load.
Specimens suitable for torsion testing can take many forms. Torsion properties can be determined from a machined metal sample, wire/rod/bar/cable/tube, or from complete products and components.
This article will focus on failure testing of round wire and rod. The torsion test will be described along with the different assessment parameters, including number of twists to failure, fracture appearance, and torque as a function of the number of twists.
The torsion test has been standardized and details can be found in 1) “ASTM A938 – 18 Standard Test Method for Torsion Testing of Wire 4 ” and 2) “ISO 7800:2012 – Metallic Materials – Wire – Simple Torsion Test 5 .”
Torsional ductility in high carbon steel products is fascinating in that it can be a function of a number of factors, including steel wire rod characteristics, wire drawing practice, heat treatment and test condition.
The torsion test has evolved over time. The test was originally developed as a ductility test where wire samples were
Rick Gordon is the technical director for SMI. He is available to help SMI members and non-members with metallurgical challenges such as fatigue life, corrosion, material and process-related problems. He is also available to help manage and oversee processes related to failure analysis. This includes sourcing reputable testing labs throughout North America, forwarding member requests to the appropriate lab and reporting results and recommendations. He can be reached at c.richard.gordon@gmail. com or 574-514-9367.
Torsion loading
Axial tensile loading
Figure 1. Manually operated torsion test unit showing important test parameters.
Figure 2. Photograph of universal drill type chucks and a sketch from ASTM A938 showing permissible 90° bends at the ends of a tests pecimen.
Figure 3. Commercially available torsion test units: a) Instron (www. instron.us/) bench top torsion test unit and b) a free standing Sjogren (www.sjogren.com/) torsion test unit.
twisted to failure. Product specifications were developed with a minimum specification limit for the number of twists. The test revealed material imperfections due to chemical segregation, surface laps and seams in the incoming rod, and imperfections introduced during the wire drawing process which could affect final product performance.
Figure 1 shows the key elements of a historical torsion testing unit. These include the gauge length, fixturing to apply an axial tensile load, and handle to apply the torsional twisting load.
Universal drill type chuck grips were introduced as method to effec tively secure the sample. A provision was added in ASTM A938 to allow bending the ends of the torsion test sample to reduce slippage for higher strength and larger diameter samples. Both are shown in Figure 2.
Commercially available torsion test units are shown in Figure 3.
Wire fracture appearance was added as an assessment criterion when it was realized in some cases that product performance did not correlate with the twist count alone. Figure 4 shows photographs of two types of torsion fractures.
A B Figure 4. 1.00 mm dry drawn wire showing: A) a completely flat ductile torsion test fracture surface (desirable) and B) a delaminated helix (undesirable) 6 .
With the development of sensor technology, torque measurements could be made during the test which lead to an improved understanding of the fracture process during testing. In addition, torsional properties of materials could be characterized such as: • Modulus of elasticity in shear • Shear yield strength • Ultimate shear strength
Figure 5 shows a torsion test unit instrumented with a torque sensor. The insert shows typical torque-twist output from the test for two specimens. The Wire Association International (WAI) produced a series of videos as part of their HardWIRED educational video program. A “Wrap Testing and Torsion Twist Testing” video 7 was produced, which shows both tests for wire. Figure 6 shows an image from the video at the end of a torsion test. For illustration purposes, a black line was marked along one side of the sample before testing. It is interesting to note the uniformity of spacing between the helical lines as well as the perpendicular (desirable) flat fracture.
Specification Review
The torsion test is described in “ASTM A938-18 Standard Test Method for Torsion Testing of Wire 4 ”and “ISO 7800:2012 Metallic Materials – Wire – Simple Torsion Test 5 ” which address the requirements for simple torsion (twist) testing of metallic wire. Both testing standards require that the wire specimen be twisted about the longitudinal axis until it fails. Upon failure, the number of turns or twists to failure are recorded.
Figure 5. Torsion test unit instrumented with a torque sensor with typical torquetwist output shown in the insert.
Figure 6. Image from the WAI wrap testing and torsion twist testing video 7 showing the flat fracture surface and uniform twist.
Figure 7. Schematic representation showing the influence of a) sulfur and b) nitrogen on torsional ductility for high carbon steel wire.
ISO 7800 includes sketches of various fractures in order to characterize fracture categories.
In a review of 15 ASTM standards published for cold formed springs produced from carbon and alloy steels and stainless steels, only two included the simple torsion test/modified torsion test (twist test): 1. “ASTM A228/A228M-18, Stan dard Specification for Steel
Wire, Music Spring 8 ” includes a torsional ductility requirement (number of twists to fracture and fracture appearance) and 2. “ASTM A230/A230M-19 Standard
Specification for Steel Wire, Carbon
Valve Spring Quality 9 ” has a twist requirement, which is a modified torsion test where only the fracture appearance is assessed.
Information developed over several decades in pursuit of improved torsional ductility yielded the following factors which can affect torsional ductility: The raw material—hot rolled wire rod used to produce wire (composi tion including element targeting and residual elements, segregation, microstructure, surface imperfec tions such as laps and seams) Wire drawing die practice (reduction per pass and wire drawing die geometry) Intermediate patent heat treatment (microstructure) Wire drawing lubrication (surface and temperature) Cooling during and after wire drawing (temperature) Torsion test condition (twist rate)
In general, it was observed that torsional ductility decreases with increasing strength and increasing wire diameter.
Examples
Two significant examples discussed in the 2019 SMI Metal Engineering eXpo presentation 3 which were found to impact torsional ductility were: 1.
2. Hot rolled wire rod composition — residual elements sulfur and nitrogen and Wire drawing die geometry – wire drawing die approach angle.
Steel Residual Element Effects
In the “Springmaking Materials/ Materials Design Philosophy” article published in the Fall 2019 10 issue of Springs, the effect of element compositions in carbon, alloy and stainless steels were discussed. Among those elements, sulfur and nitrogen were found to have a significant impact on torsional ductility.
Figure 7 shows schematic representations for three wire diameters between torsional ductility of high carbon steel wire and sulfur composition or nitrogen composition. Lower levels of each element show higher torsional ductility values. In general, this is because sulfur is related to the size and distribution of inclusions (dirt) in the steel and nitrogen affects the strain aging sensitivity of the steel.
Carbide die or nib
Bell radius
Entrance angle
Approach angle Steel casing
Back relief Bearing or land
Figure 8. Cross section of a tungsten carbide wire drawing die.
30 Torsional Ductility (Twists to Failure - 8 in. Gage Length) 18 20 22 24 26 28 16
4 5 6
7 8 9 10 11 12 Wire Drawing Die Approach Angle (degrees)
13 14 15
Figure 9. The effect of wire drawing die approach angle on torsional ductility of 0.093 in. diameter high carbon steel wire.
For the discussion of wire drawing die geometry, some definitions are necessary. Figure 8 shows a cross section of a tungsten carbide wire drawing die with important features identified.
The following section provides a brief description of the features.
“Wire drawing takes place in a tungsten carbide nib which is contained typically in a carbon steel case. The die entry geometry, which includes the bell and entrance angles, is designed to promote wire drawing lubricant flow and alignment and reduce abrasion. The approach angle (one of the most important features of the die and discussed below) establishes the deformation zone where the actual size reduction takes place. The bearing section is designed to preserve the drawn wire size. The back relief is designed to minimize scraping at the die exit.”
Figure 9 shows the influence of the wire drawing die approach angle on torsional ductility of 0.093 in. diameter high carbon steel wire. Historically, an approach angle of 12 degrees has been used for high carbon steel wire draw ing. By reducing the approach angle to 8–9 degrees, a significant improvement can be realized for torsional ductility. Improvements in torsional ductility translate to a reduction of non-conforming product and can allow for the use of lower cost steels to achieve satisfactory product performance.
Summary
In this article, the torsion test was described along with key ductility measures. Torsional ductility can be a function of many factors involving the raw material, the wire drawing and heat-treating processes, and testing conditions. Torsional ductility improvement ideas involving steel composition and wire drawing die geometry were described.
Moving Forward
The bend, reverse bend and hardness tests are planned for review in future articles.
Future Work
I believe the torsion test should be included in other ASTM spring wire standards as it is a recognized test for characterizing the ductility of wire products. Through the inclu sion of the torsion test as a product testing requirement for other ASTM spring wire standards, I believe this represents a possible opportunity for springmakers to assure a more uniform product is received. I plan to discuss this at a future ASTM subcommittee meeting. n
References
1. Gordon, C.R., Types of Tests Used to Characterize Springmaking Materials –Part 1: The Tensile Test, Springs, Winter 2020, p.27
2. Gordon, C.R., Types of Tests Used to Characterize Springmaking Materials –Part 2: The Coiling and Wrapping Tests, Springs, Spring 2020, p. 27
3. Gordon, C.R., Torsional Ductility as an Important Quality Parameter for Spring Making Materials, SMI Metal Engineering Expo 2019, https:// www.metalengineeringexpo.org/ wp-content/uploads/2019/10/ Torsional-Ductility-as-an-Important-Quality-Parameter-SMI-Gordon-092319.pdf
4. ASTM A938 – 18 Standard Test Method for Torsion Testing of Wire (replaced E558) (https://www.astm.org/)
5. ISO 7800:2012 – Metallic Materials –Wire – Simple Torsion Test (replaced ISO R136)
6. W. Van Raemdonck et al., “Torsion Tests as a Tool for High Strength Wire Evaluations,” Steel Cord Technical Report, Wire Association International (1993), p.87.
7. Wrap Testing and Torsion Twist Testing Video, https://www.youtube.com/ watch?v=XuG9zL4SBGk.
8. ASTM A228/A228M-18 Standard Specification for Steel Wire, Music Spring Quality
9. ASTM A230/A230M-19 Standard Specification for Steel Wire, Carbon Valve
Spring Quality 10. Gordon, C.R., Springmaking Materials / Materials Design Philosophy, Springs,
Fall 2019, p. 23.
