Hardness is a measure of the resistance of a steel to indentation and is related to the maximum strength of the steel.

When steel undergoes heat treatment, its properties are altereted after being subjected to a series of temperature changes. The steel is heated and then cooled where necessary to provide changes in its structural form and its hardness, strength or ductility may all be increased through these changes. The time spent at each temperature and the rates of cooling have significant impact on the effect of the treatment.

High speed steel refers to a range of metal-cutting tool steels that retain their hardness at red heat. Key properties include high working hardness, wear resistance and toughness, good compressive strength and an ability to perform at temperatures up to 5000C without losing hardness. The main use of high-speed steels is for the manufacture of cutting tools such as drills, milling cutters, gear cutters, saw blades etc.

HMS stands for heavy melting scrap, and this is composed of two different categories. The main difference between the two is that HMS1 does not contain galvanized and blackened steel unlike HMS2. Because both grades guarantee a minimum piece thickness – at least 1/4inch (6.3mm) for HMS 1, and 1/8in for HMS 2 – consignments have a high density. Both also have defined maximum dimensions (usually 60in x 24in), and should be prepared to facilitate handling and charging to a furnace. This density, sizing and preparation makes for efficient furnace operation by minimising the time to charge enough scrap for a full melt. In contrast, thin mixed scrap greatly increases charging time, cutting furnace productivity.

Hot briquetted iron describes direct reduced iron that has been processed into briquettes. Instead of using a blast furnace, the oxygen is removed from the ore using natural gas and results in a substance that is 90%–92% iron. Because DRI may spontaneously combust during transportation, HBI is preferred when the metallic material must be stored or moved.

Hot end refers to stages in steelworks where metal is either molten, or is solid but can only be worked at elevated temperatures. Long products are generally hot rolled to their final shape.

Hot idling is a procedure which temporarily maintains a furnace on standby without producing any iron. To slow the combustion and maintain the refractory lining, the amount of air which enters the furnace is significantly reduced. The main chamber is kept fully charged with coke (but not with limestone and iron ore, the two other bulk ingredients for iron making). Blast furnaces are not usually hot idled for more than a few weeks unless all the iron is completely drained from the furnace, and they can be returned to full iron making capacity within a matter of days.

A hot-strip mill is essentially a rolling mill made up of several stands of rolls. These convert slabs into hot-rolled coils. The hot-strip mill squeezes slabs, which can range in thickness from two to ten inches, depending on the type of continuous caster, between horizontal rolls with a progressively smaller space between them (while vertical rolls govern the width) to produce a coil of flat-rolled steel about a quarter-inch in thickness and a quarter mile in length.

A forming process in which a tube is placed into a forming die. The tube is then formed to the shape of the die through the application of internal water pressure. The hydroforming process allows for severe shape deformation, making it ideal for automotive structural parts such as engine cradles, radiator supports, and body rails. Various shaped and sized holes can be punched in the tube almost anywhere during the process.