| Machinability | Simply defined as a measure of the ease with which a metal can be machined satisfactorily. |
| Macroscopic Stresses | Residual stresses that vary from tension to compression in a distance (presumably many times the grain size) that is comparable to the gage length in ordinary strain measurements, hence, detectable x-ray or dissection methods. |
| Macrostructure | The general crystalline structure of a metal and the distribution of impurities seen on a polished or etched surface by either the naked eye or under low magnification of less than x10. |
| Magnetic Crack Detection | The bar or component to be tested is magnetized by passing a heavy current through it or by making it the core of a coil through which a heavy current is passed. Cracks or inclusions cause the magnetic flux to break the surface forming free magnetic poles. When the component is sprayed with a suspension of finely divided magnetic particles they collect at the free poles to visibly show the presence of defects. |
| Malleability | It can be defined as the property of a metal to be deformed by compression without cracking or rupturing. The load may be applied slowly or suddenly and will determine whether the material will be suitable for forging or rolling into thin sheet. |
| Malleable Cast Iron | A cast iron made by prolonged annealing of white cast iron in which decarburization or graphitization, or both, take place to eliminate some or all of the cementite. The graphite is in the form of temper carbon. If decarburization is the predominant reaction, the product will exhibit a light fracture surface, hence, "whiteheart malleable;" otherwise, the fracture surface will be dark, hence, "blackheart malleable." Ferritic malleable has a predominantly ferritic matrix; pearlitic malleable may contain pearlite, spheroidite or tempered martensite depending on heat treatment and desired hardness. |
| Manganese | One of the most important constituents of steel in which it fulfils a number of functions. It acts as a mild de-oxidizing agent. It combines with the sulfur present to form globular inclusions of Manganese Sulfide which are beneficial to machining. It increases tensile strength and the hardenability of steel. |
| Maraging | A precipitation-hardening treatment applied to a special group of iron-base alloys to precipitate one or more intermetallic compounds in a matrix of essentially carbon-free martensite. |
| Martempering | A heat treatment involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the Ms point. Soaking must be long enough to avoid the formation of bainite. The advantage of martempering is the reduction of thermal stresses compared to normal quenching. This prevents cracking and minimizes distortion. Also called Marquenching. |
| Martensite | The hard constituent produced when steel is cooled from the hardening temperature at a speed greater than its critical cooling rate. Martensite is an acicular phase when seen in the microstructure of steel. |
| Martensitic Transformation | A reaction that takes place in some metals on cooling, with the formation of an acicular structure called martensite. |
| Mass Effect | A term used to signify the effect of size and shape during heat treatment, since it is the rate of cooling of a piece of steel which determines the properties resulting from the hardening and quenching process. |
| Matrix | The mass or principal constituent (e.g. iron in the case of steel) in which other constituents are embedded. |
| Maximum Stress | In the testing of the strength of steel a sample is machined into a standard test piece and is stretched in a tensile testing machine until it breaks. The results are expressed in N/mm2 and is the value of the maximum load reached in the test divided by the original cross sectional area of the specimen. |
| McQuaid EHN Grain Size Test | A method of assessing grain size. It consists of a test piece at 927°C for 8 hours by slow cooling and subsequent microscopical examination. The grain size is measured at x100 magnification and compared to standard charts, the figures range from No. 1 - very coarse, to No. 8 - very fine. |
| Mechanical Properties | The properties of a material that reveal its elastic and inelastic behavior when force is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile strength, elongation, hardness, and fatigue limit. Compare with physical properties. |
| Meehanite | A trade name applied to a certain type of cast iron. |
| Melting Point | The temperature at which a solid begins to liquefy. |
| Metallizing | (1) Forming a metallic coating by atomized spraying with molten metal or by vacuum deposition. Also called spray metallizing. (2) Applying an electrically conductive metallic layer to the surface of a nonconductor. |
| Mg | Chemical symbol for Magnesium. |
| Micro Segregation | Segregation within a grain, crystal or small particle. |
| Microhardness | The hardness of a material as determined by forcing an indenter such as a Vickers or Knoop indenter into the surface of a material under very light load; usually, the indentations are so small that they must be measured with a microscope. Capable of determining harnesses of different microconstituents within a structure or of measuring steep hardness gradients such as those encountered in case hardening. |
| Micron | A unit of length equal to one millionth of a meter (0.001mm). |
| Microstructure | The structure that is observed when a polished and etched specimen of metal is viewed in an optical microscope at magnifications in range of approximately x25 to x1500. |
| Mill Scale | The heavy oxide layer formed during hot fabrication or heat treatment of metals. |
| Mn | Chemical symbol for Manganese. |
| Mo | Chemical symbol for Molybdenum. |
| Modulus of Elasticity | This is a measure of rigidity based on the ratio of stress to corresponding strain in an elastic material. When a material is subjected to an external load it becomes distorted or strained. With metals, provided the loading is not too great, they return to their original dimensions when the load is removed, i.e. they are elastic. Within the limits of elasticity, the ratio of the linear stress to the linear strain is termed the modulus of elasticity or more commonly known as Young’s Modulus. |
| Modulus of Rupture | Nominal stress at fracture in a bend test or torsion test. In bending, the modulus of rupture is the bending moment at fracture divided by the section modulus. In torsion, modulus of rupture is the torque at fracture divided by the polar section modulus. |
| Molybdenum | Element, chemical symbol Mo. A grayish metal that does not easily tarnish in air and has a high melting point of 2620°C (4748°F). Its use as an alloying element in steel increases hardenability and in low alloy steels reduces the risk of temper brittleness. When added to stainless steels it increases their resistance to corrosion. It is also used in high-speed steels. |
