Laboratory 3: Quenching and Hardenability | MAT E 443, Lab Reports of Materials science

Download Laboratory 3: Quenching and Hardenability | MAT E 443 and more Lab Reports Materials science in PDF only on Docsity! Mat E 443 - Laboratory #3: Quenching & Hardenability Overview In this laboratory exercise, laboratory teams will water quench and oil quench round bar specimens of 1045 steel of varying diameter. Hardness profiles will be measured to estimate the critical diameter for these two quench media. Background The ease with which a particular steel can be quenched to form martensite (i.e. hardened) is commonly referred to as the property “hardenability”. Because this definition does not lend itself to quantification, an alternative, more useful, definition of hardenability is the depth to which a particular steel is hardened upon quenching. Even this definition, however, has considerable ambiguity. Specifically, the term “hardened” is not well defined and the phrase “upon quenching” does not provide any particular detail about the quenching medium (or process). Accordingly, we could further specify the hardenability as the depth to which some fraction (e.g. 50%) martensite is formed when quenched in a particular medium. While this definition is more clear, the measurement of such a quantity would require excessive microstructural observation and measurement to determine the location of the transition through 50% martensite. Since hardness varies directly with percent martensite, the 50% martensite location can be determined rather easily by measuring a hardness profile through the cross section of a quenched part. However, the hardness at 50% martensite varies considerably with carbon content, from HRC 40 for 1040 to HRC 54 for 1080, as shown below. A summary of the measured hardness (HRC) as a function of carbon content for plain carbon steel quenched to 50% martensite.1 To make the above definition even more practical, we remove the influence of part geometry and define the property critical diameter (dc) as a measure of hardenability. The critical diameter is 1 M.A. Grossman and E.C. Bain, Principles of Heat Treatment, 5 th ed. American Society for Metals, 1964. defined as the diameter of round bar that is hardened to 50% martensite (or greater) through its entire cross section upon quenching in a particular medium. The remaining problem with the definition above is that the critical diameter is not a material property because it depends on the quench medium. Different quench media vary considerably with respect to their effectiveness in quenching. This is primarily due to their convective properties, and is quantified by the Grossman number (H), which expresses the ratio of heat transfer due to quenchant convection to that associated with diffusion from the steel to the quenchant. Higher Grossman number indicates more dominate convective transfer and a more effective quench. For this reason, the Grossman number is also known as the quench severity. Typical quench media Grossman numbers are shown below. Grossman numbers (H) showing the severity of quench for common media2 Upon quenching a steel in a very ineffective quench medium (low H), the temperature of the steel at the surface would decrease and the temperature of the quenchant at the interface would increase, and heat removal would depend on thermal diffusion through both the steel and a long boundary layer of quenchant. In a very effective quench medium (high H), fluid convection would decrease the thickness of the boundary layer to a very small value. A useful exercise is to compute the critical diameter for a quench medium that is ideal, where the boundary layer 2 M.A. Grossman and E.C. Bain, Principles of Heat Treatment, 5 th ed. American Society for Metals, 1964. Critical diameter vs ideal critical diameter for media of different quench severity.2