Testing to Determine Material and Mechanical Properties (Part One: The Tools)
Having the right tools in our toolbox or knowing who to call and rely on makes any job go easier, faster and better. In the analysis of heat-treated parts, we often must call on outside testing laboratories or universities for help. The Doctor is often asked questions such as: “What tools do these sources have?” “What tests should I have them run?” “How can they help us get the answers we need?” Let’s learn more.
Universal Testing Machine Investigations
Universal testing machines (Fig. 1) fall within the general class of mechanical-testing devices that were traditionally used to test the tensile and compressive strength of materials. Depending on the equipment setup, however, one can examine samples after stretching, compression, bending, twisting and the like.
These types of machines are used to determine the basic strength and plasticity values of samples after various heat-treatment processes, including: tensile strength, compressive strength, flexural strength (i.e., resistance to bending), modulus of elasticity, yield (and offset yield) strength, limit of elasticity, elongation and reduction of area.
By far, the most-used and well-known impact test is the Charpy test, which can determine a material’s resistance to cracking under dynamic load. These tests measure the total amount of energy that a material is able to absorb. This energy absorption is directly related to the brittleness of the material. Therefore, one can determine such things as the ductile-to-brittle transition temperature. Understanding a material’s energy-absorption properties is critical because it predicts how much plastic deformation the material will be able to withstand before catastrophic failure.
Hardness testing is the most common heat-treatment test performed throughout industry due in large part to its speed and simplicity. Hardness, in simplest terms, is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. It is also valuable because it reveals a relationship with certain mechanical properties, so it can be used to approximate tensile strength without conducting a tensile test.
In the laboratory, macro- and microhardness testing is typically performed for a variety of reasons, including checking the accuracy of field results. Hardness on a macro scale employs loads above 10 N (1 Kg). The most common methods include:
- Brinell – With a load that can be applied over a wider surface area than other test methods, it is often used to test cast irons and soft materials (e.g., to measure materials prior to heat treatment or those that have been annealed or normalized). It consists of pressing a penetrator into the material. A steel ball is used for research of soft materials and sintered carbide for the harder ones. The bead diameters may be 1, 2, 2.5, 5 and 10 mm,
and loads may range from 10-3,000 N.
- Rockwell – This is a fast method often used to determine a successful heat treatment. The measurement consists of applying force in two stages: an initial force (preload) to set the indenter followed by the main force. Hardness is determined on the basis of the difference of impressions for individual loads. The three types of indenters include a diamond cone with a tip angle of 120 degrees (for measurements with scale A, C, D), a steel ball with a diameter of 1.588 mm (for measurements with scale B, F, G) and a steel ball with a diameter of 3.175 mm (for measurements with scale E, H, K).
- Vickers – This is considered the most versatile of microhardness test methods. It can be used for virtually any material, from very soft to very hard. The test involves pressing the pyramid-shaped penetrator (with a gap angle of 136 degrees) into the material, with a given load that can vary from to 1-10 N (0.1-1 Kg).
- Knoop – An alternative microhardness measurement to Vickers, the only difference is the shape of the penetrator (indenter), which is an elongated diamond shape. A key advantage of Knoop testing is that the shape of the indenter enhances the ability to measure closer to the surface and measure thin layers or coatings.
- Low-load testing – Light loads can be used to determine hardness distributions of surface layers after surface treatments. It may be performed by either Vickers or Knoop methods and typically involves loads between 1-3 N (0.1-0.3 Kg). It is important to note that no conversion of these measurements to other scales (particularly Rockwell “C”) is allowed.
The microscope is to the materials engineer what the hammer is to a carpenter. Their usefulness and versatility in metallurgical investigations is fundamental. The most common types include:
- Stereo microscopes – Use magnifications of 10-50X for examination of surface features/defects, fracture surfaces, braze joints and other basic macroscopic features. More sophisticated units can provide topographic (3D) mapping of surfaces.
- Optical microscopes – These now have extended magnification capability, up to 2000X. Metallographically prepared samples are required (i.e., properly mounted, polished and etched specimens).
- Scanning electron microscopes (SEM, TEM, STEM) –
These (Fig. 2) have magnification capability up to 250,000X. Use of higher magnification reveals features on structural components far beyond optical microscopy. They also have an additional advantage, namely that when the appropriate detectors (e.g., EDS and EELS) are installed, the chemical composition of the material can be qualitatively or quantitatively determined. Due to the very large depth of field, they are used extensively for failure analysis and forensic metallurgy work.
- Image analysis, digital cameras – Software and hardware packages are available for the various microscopes to perform such tasks as automatically detecting objects, layers, area fractions or optical profiles for data collection as well as display/photography of microstructures.
Chemical composition tests are used for the general (average) chemical composition via traditional (wet) chemical analysis or via spectrometers (portable or cabinet), including:
- Fluorescent X-ray spectrometers – These allow rapid determination of the composition of an unknown sample using the emissions of characteristic X-rays. The available range includes elements from sodium to uranium. Depending on the method of the test and the material being tested, it is possible to detect constituents to 0.0001%.
- Optical spectrometers (Fig. 3) – These are used to determine the elements present in a material. Rely on the fact that each element, after energy excitation and volatilization, emits electromagnetic radiation, which contains only certain wavelengths characteristic of the elements present. The excitation of the sample may be caused by an electric spark with a direct- or alternating-current arc, a glow discharge, a laser discharge or a plasma torch.
- Analyzers using infrared absorption spectrometry
(Fig. 4) – These are used for the analysis of carbon, sulfur, phosphorus, nitrogen and hydrogen in metals.
- FTIR (Fourier Transform Infrared Spectroscopy) – This is an analytical technique used to identify organic, polymeric and (in some cases) inorganic materials using infrared light to scan test samples and observe chemical properties.
- Changes in chemical composition – Used most often to determine changes in highly localized (micro) areas (~~1 µm).
- Via X-ray analysis: The principle of operation is similar to fluorescent X-ray spectrometers but is capable of bit-mapping to show the distribution of elements on the observed surface as well as distributions along selected spectral lines. This is especially useful for analyzing elements after thermo-chemical treatments or with applied coatings.
- Via optical spectroscopy: The material is evaporated as a result of excitation, and a crater with a depth of up to 100 µm is formed on the surface. The vaporized elements are analyzed and their concentration determined. Some spectrometers are equipped with special software allowing them to analyze the chemical composition during the entire crater formation, which allows determination of changes in the chemical composition as a function of depth into the material.
A wide variety of highly specialized test equipment is available to determine corrosion or wear properties, residual stress states, retained-austenite levels, contamination, etc. Two examples are:
- Gleeble® test machines – These perform physical simulation of material processing involving the exact reproduction of the thermal and mechanical processes in the laboratory that the material is subjected to in the actual fabrication or end use. Depending on the capability of the machine performing the simulation, the results can be used to determine such items as CCT or TTT diagrams (e.g., Ms or Bs temperatures), thermal fatigue, hot ductility, stress versus strain and creep/stress rupture.
- Auger analysis – This is a common analytical technique used specifically for the study of localized areas of interest, such as elemental chemical mapping of ultra-thin surfaces (in the order of 40 Å per pass). It uses the analysis of energetic electrons emitted from an excited atom to determine such items as surface contamination, material chacterization and the like.
The key to using any outside resources lies in clearly understanding what tests will help answer the question(s) you have or reveal the root cause of a problem you are investigating. As a result, good communication of the intended end-use component application (including the material and design requirements), its heat treatment and/or the specific problem you are trying to investigate or understand is at the heart of working with outside laboratories or universities.
The reason this is so important is that you want to obtain test results you know are both accurate and representative and ones you can interpret to make informed decisions. Data alone or conclusions without supporting facts are most often useless.
Next time: We present several case studies to illustrate how these tools are used in solving heat-treat problems.
- Professor Leszek Klimek, Institute of Materials Science and Engineering, Łódz´, University of Technology, technical contributions
- Professor Emilia Wołowiec-Korecka, Łódz´ University of Technology, Łódz´, Poland, technical contributions and private correspondence
- Herring, Daniel H., Atmosphere Heat Treament, Volume II, BNP Media, 2015
- “Charpy vs. Izod: An Impact Testing Comparison,” Element (www.element.com)
- Herring, Daniel H., “A Comprehensive Guide to Heat Treatment, Volumes 1 & 2,” Industrial Heating, 2018 (e-books)