Improving the strength/toughness relationship increases the usefulness of any engineered material. Steel is no exception.
|Fig. 1. Relative toughness (Gc) of various materials|
Improving the strength/toughness relationship increases the usefulness of any engineered material (Fig. 1). Steel is no exception. As heat treaters, we need to understand what influences steel toughness and ask ourselves what tests best measure it. Let’s learn more.
What is Toughness?
Toughness is a fundamental material property measuring the ability of a material to absorb energy and withstand shock up to fracture; that is, the ability to absorb energy in the plastic range. In other words, toughness is the amount of energy per unit volume that a material can absorb before rupturing and is represented by the area under the (tensile) stress-strain curve (Fig. 2). In service, this loading often occurs in the form of (sudden) impact.
When considering toughness, one must make the distinction between impact toughness, which most often occurs under high strain-rate loading above the yield point, and fracture toughness, which generally occurs under lower strain-rate loading. All steels have different strength and ductility characteristics as a function of their composition (i.e. alloy design), and the key to good toughness is a balance between these properties and overall life-cycle cost.
Tough materials can absorb a considerable amount of energy before fracture, while brittle materials absorb very little. Comparing areas under each stress-strain curve reveals this difference. A material with high strength and high ductility will have more toughness than a material with low strength and low ductility. Recall that brittle materials may be strong, but they are not tough due to limited strain values.
Why is Toughness Important?
Catastrophic failures are caused by a combination of inadequate material properties, improper design, poor manufacturing or fabrication processes, uneven or excessive loading, and pre-existing flaws. These failures can most often best be addressed by understanding the strength and toughness characteristics of a given material, including both its impact and fracture toughness.
|Fig. 3. Effect of tempering temperature on impact properties|
What Influences Toughness of a Material?
The general factors that influence toughness are alloying elements, fabrication techniques, microstructure, temper condition and service application (e.g., temperature, strain rate, strength-to-ductility ratio and the presence of stress concentrators).
There are several variables that have a profound influence on the toughness of a material. These include rate of loading (i.e. strain rate), temperature, distribution of stress, surface topography, and the presence or absence of any pre-existing flaws or cracks and their stress intensity.
A steel may possess satisfactory toughness under static loads but may fail under dynamic loads or impact. In general, ductility and, as a consequence, toughness decrease as the rate of loading increases. Most materials are more brittle at lower temperatures and more ductile at higher temperatures. Finally, the distribution of stress is critical. A material might display good toughness when the applied stress is uniaxial, but when a multiaxial stress state is produced due to the presence of a notch the material might not withstand the simultaneous elastic and plastic deformation in the various directions.
As a general rule, the lower the hardness and strength, the higher the ductility and toughness of a microstructure. However, embrittlement phenomena (e.g., quench embrittlement, temper embrittlement) are exceptions to this rule. For example, tempered martensite embrittlement (TME) lowers ductility and toughness as hardness decreases within a particular range of tempering temperature (Fig. 3). This is why after tempering of certain alloy steels such as 4140 or 4340 at temperature of 480-750°F (250-400°C) the impact toughness is lower than that obtained on tempering at temperatures below 480°F (250°C) or above 750°F (400°C).
In crystalline materials, the toughness is strongly dependent on crystal structure. Face-centered-cubic (FCC) materials are typically ductile, while hexagonal-close-packed (HCP) materials tend to be brittle. Body-centered-cubic (BCC) materials often display dramatic variation in the mode of failure with temperature. Steels with ferritic microstructures have inherently lower toughness when tested below their ductile-to-brittle transition temperatures.
|Fig. 4. Impact toughness as a function of temperature|
The toughness of a material can be measured by tensile testing, where the total area under its stress-strain curve measures, at low strain rates, reduction of area and total elongation – both parameters sensitive to fracture. This value, the so-called material toughness, equates to a slow absorption of energy by the material, and it has units of energy per volume.
A comparison of the relative magnitudes of the yield strength, ultimate tensile strength and percent elongation of different material will give a good indication of their relative toughness. Materials with high yield strength and high ductility have high toughness. Integrated stress-strain data is not readily available for most materials, so other test methods have been devised to help quantify toughness (Fig 4).
The most common test for toughness is the Charpy V-notch impact test (previously Izod testing), which evaluates the effect of high strain-rate loading and a sharp notch on the energy absorbed for fracture. Impact toughness is the ability that a material possesses to absorb energy either with or without the presence of a stress intensifier such as a notch. Since toughness is greatly affected by temperature, these tests are often repeated several times with each specimen tested at a different temperature. The ductile-to-brittle transition temperature (DBTT) is often an extremely important consideration in the selection of a material. The use of alloys below their transition temperature is avoided due to the risk of catastrophic failure.
Fracture toughness (K1c) testing evaluates stress intensities required to propagate unstable fracture in front of a sharp crack under conditions of maximum constraint of plastic flow. It is an indication of the amount of stress required to propagate a pre-existing flaw due to processing, fabrication or end-use application. Flaws may appear as cracks, voids, non-metallic inclusions, weld defects, hot shortness, discontinuities, overheating during forging or a combination of several types. Fracture toughness generally depends on temperature, environment, loading rate, the composition of the material and its microstructure, together with geometric effects (constraints). Although it is possible to correlate Charpy energy with fracture toughness, a large degree of uncertainty is associated with correlations because they are empirical.
It is preferable to determine fracture toughness in a rigorous fashion, in terms of K (stress intensity factor), CTOD (crack tip opening displacement) or J (the J integral). ASTM E1290 covers CTOD testing, ASTM E1820 includes K, J & CTOD (including R-curves) and ASTM E1921 covers J testing to determine T0 for ferritic steels.
Being a complex subject, there is more to discuss, such as the influence of alloying elements (e.g., carbon, nickel), microstructure, heat treatment, embrittlement phenomena and service conditions on toughness. These subjects will be discussed in more detail in an upcoming Heat Treat Doctor column (March 2011). IH