What is spring heat treatment? As some may know, spring heat treatment is a metalworking process that uses heating, cooling, and holding at specific temperatures to achieve the desired microstructure and properties of the material. The heat‑treatment process can be divided into four main steps; below, we’ll take a closer look at what those steps are in the heat treatment of hardware springs.

　　I. Classification of Spring Heat Treatment Processes

　　Spring heat‑treatment processes can broadly be classified into three major categories: through‑hardening, surface hardening, and chemical heat treatment. Depending on the heating medium, heating temperature, and cooling method, each of these categories can be further subdivided into several distinct heat‑treatment processes. For a given spring, employing different heat‑treatment procedures yields varying microstructures, which in turn confer different mechanical properties. Steel is the most widely used metal in industry, and its microstructure is also among the most complex; consequently, there exists a vast array of steel heat‑treatment processes.

　　Overall heat treatment is a metal‑working process in which the entire workpiece is heated and then cooled at an appropriate rate to obtain the desired microstructure, thereby modifying its overall mechanical properties. For steels, the four principal types of overall heat treatment are annealing, normalizing, quenching, and tempering.

　　II. Heat Treatment Process for Springs

　　The heat‑treatment process for springs typically comprises three stages—heating, holding at temperature, and cooling—though in some cases it involves only heating and cooling. These stages are sequentially linked and must be carried out without interruption.

　　Spring heating is one of the crucial processes in heat treatment. There are numerous methods for heating springs during heat treatment; historically, wood charcoal and coal were used as heat sources, followed by liquid and gaseous fuels. The adoption of electric heating has made temperature control more precise and eliminated environmental pollution. These heat sources can be employed either directly to heat the workpiece or indirectly, via molten salts or metals, or even through suspended particles.

　　During spring heating, workpieces exposed to air often undergo oxidation and decarburization—i.e., a reduction in carbon content at the surface of steel parts—which can severely compromise the surface properties of the components after heat treatment. Therefore, metals are typically heated in a controlled or protective atmosphere, in molten salts, or under vacuum; alternatively, protective coatings or packaging methods may be employed to prevent undesirable surface changes.

　　Heating temperature is one of the key process parameters in heat treatment; selecting and controlling it is essential for ensuring the quality of the treatment. The appropriate heating temperature varies depending on the metal material being processed and the specific objectives of the heat treatment, but it is generally raised above the phase transformation temperature to obtain a high‑temperature microstructure. Moreover, phase transformations require a certain amount of time; therefore, once the surface of the workpiece has reached the desired temperature, it must be held at that temperature for a specified period to equalize the temperature throughout the part and ensure complete microstructural transformation. This holding period is referred to as the soaking time. When using high‑energy‑density heating or surface heat treatments, the heating rate is extremely rapid, so a separate soaking time is usually not required; by contrast, chemical heat treatments often involve relatively long soaking times.

　　Cooling is also an indispensable step in the heat‑treatment process; the cooling method varies depending on the specific process and primarily serves to control the cooling rate. Generally, annealing employs the slowest cooling rate, normalizing a relatively faster rate, and quenching an even faster one. However, requirements also differ among steel grades; for example, air‑hardening steels can be hardened by quenching at a cooling rate comparable to that used in normalizing.