Hardened and Tempered Steel Strip
Hardened and tempered spring steel strip (H&T) has good ductility levels that facilitate the manufacturing of complex component shapes and reduce relative wear on tooling. It also exhibits excellent mechanical properties, superb flatness and outstanding spring and cutting qualities.
This is achieved by heating the steel to above its critical transformation temperature and then rapidly cooling through quenching. The result is a hard but extremely brittle material that requires further treatment in the tempering process.
Toughness
Hardened steel strip is characterized by its toughness. Toughness is defined as the ability of a material to absorb energy without fracture. It can be measured by determining the uniaxial yield strength of the material. This is achieved by placing a 6 mm radius sphere on the surface of a flat plate of the same material and loading it against it. The resulting uniaxial stress is calculated by using the formula E=2Gpa/n where the n represents the load applied and Gpa stands for the uniaxial yield strength of the material.
Our hardening & tempering lines guarantee a uniform state of stress throughout the strip. This enables us to achieve high tolerances on edge camber and flatness, making our strip well suited for a wide range of special applications.
The high spring properties and uniformity of hardness in our strips make them suitable for precision blanking, which increases the efficiency of processing the strip and reduces waste. It also reduces the need for post heat treatment and tempering, which reduces the costs involved in these operations. It also ensures the strip has a good tolerance on thickness, which is very important for many special applications. The tolerance allows the production of parts with a high level of accuracy and helps to optimize the functionality of the components. This is particularly the case with complex geometries or in areas where the edges of the component are in contact with other materials, such as when used in prosthetic limbs or for coatings like glass and plastic.
Tensile Strength
The ultimate tensile strength of hardened steel strip is determined by a number of factors including the microstructure, heat treatment and annealing conditions. The Company uses a controlled heat treatment process to produce martensite microstructures in their cold rolled and annealed strips with very high strength levels. This process is based on the principle that when a material is subjected to a hardened steel strip stress greater than its yield strength it will experience ‘critical’ grain growth. This will reduce the area of the material in contact with a load and thus increase its tensile strength.
To improve the quality of its products, and to allow it to achieve higher tensile strengths, it also uses an annealing procedure that is different from the conventional method. This involves a small amount of cold working on the strand anneal line, which allows the strain energy to generate critical grain growth, increasing the tensile strength of the material.
The Company produces a range of hardened and tempered strip in a variety of thicknesses and widths. This material is supplied with either round or square edge profiles which are advantageous in fatigue-resistance applications. It is primarily used in the manufacture of doctor blades for paper manufacturing industries, where one strip edge is machined to give a specific blade profile. In this application the Company uses a very effective in-line machining operation to provide a strip that is able to withstand demanding tolerances on edge camber and flatness, and be free from waviness.
Impact Strength
Hardened and tempered steel strip is the basic raw material from which many spring-type components are made, from clutch plate segments to washers, retractor springs in cars Stainless Hardened SteelStrip to tree springs on riding saddles. This is a particular specialization of the Company and considerable plant development has occurred to provide a very wide range of dimensions, mechanical properties and coil weights.
The hardening and tempering process is a series of continuous heat-treating operations that transform cold rolled steel into high-strength spring steel. The Company carries out this operation at the Pleasant Priarie plant using state-of-the-art, continuous, carburizing and quenching furnaces manufactured by EBNER, the world’s leading manufacturer of these systems.
These furnaces have been designed to allow for the precise control of metallurgical properties in order to achieve optimum results. During the hardening stage, the strip is heated to very high temperatures in order to make it extremely hard, but this also makes it quite brittle. The second stage, called tempering, involves heating the strip to lower temperatures so that it softens a little and develops some ductility, which helps to improve impact resistance.
A typical impact test involves loading a sphere of a fixed radius (say 6 mm) against a flat plate of the same material. A quartz force sensor under the steel plate records the transmitted forces and calculates the energy absorbed. This data is used to determine the impact strength of the material.
Corrosion Resistance
Corrosion resistance is one of the most important properties exhibited by hardened steel strip. It is the ability to withstand degradation by contact with corrosive mediums which cause corrosion damage starting at the exposed surface and gradually spreading throughout the entire metal. This deterioration is a natural process which can be mitigated by appropriate alloy additions and careful control of the heat treatment.
Stainless steels are generally considered to have superior corrosion resistance as compared to carbon and low-alloy irons. This is partly due to the presence of chromium which enhances the resistance to pitting and crevice corrosion by increasing the formation of the passive film. Other alloying elements such as molybdenum and nitrogen also increase corrosion resistance in some grades.
High-performance grades combining martensite and precipitation hardening achieve good strength and toughness by the addition of nickel, vanadium, boron, and cobalt. These steels are often used in aerospace applications and are normally supplied in a non-oxidized (blue-black or blue-grey) finish which is acceptable for most engineering components.
By carefully controlling the hardening and tempering conditions it is possible to produce a wide range of very specific engineering materials. For example, the Company can supply sheared and edge-dressed strip to a high standard for use in applications like automotive clutches where a round profile is preferred for fatigue resistance benefits.