ROQLAST TH 360 ROQ-Last abrasion resistant steel plates are available in two quenched and tempered grades produced to give an optimal combination of hardness, toughness and weldability. The steel is made to fully killed fine grain practise. It is calcium treated and desulphurised to achieve low sulphur levels and a very low inclusion content with controlled shape. These processes result in the steel having adequate toughness in addition to high hardness. The plates are heated in a roller quenching plant which subjects the entire plate to a rapid, high volume water quenched. The high cooling rate ensures maximum exploitation of the alloying elements to give the required properties throughout the thickness of the plate. In this way the composition can be kept sufficiently lean to provide a readily weldable product. All plates are ultrasonically tested prior to heat treatment. Care should be taken to distinguish between ROQ-last and ROQ-tuf quenched and tempered steel grades which have similar chemical compositions. ROQ-last is quenched and subsequently tempered at lower temperatures or depending on the grade supplied as quenched, which gives it a high hardness for wear resistance, but reduced toughness which is nevertheless adequate for such applications. The higher tempering temperature also results in lower tensile strength and hardness. ROQ-last TH360 in thickness upto 32mm and ROQ-last TH400 were however designed to be suitable for both wear and limited structural applications. Chemical Composition
Grade Selection The harder grades give better resistance to sliding abrasion. In these cases toughness is of secondary importance. In other applications where toughness is important, steel grades with lower hardness should be used. The selection of a particular grade will therefore depend on the intended application. Hardness Hardness of heat treated steel relates to the condition at the surface, whereas hardenability of the steel grade is a measure of depth to which the hardness is maintained in the steel. To obtain the required hardness in thick plate, it is essential to increase the ability of the steel to be hardened. Hardenability of a steel grade is determined by the chemical composition. As the thickness of steel plates is increased, the alloy content to be increased to obtain the required hardenability. Mechanical Properties The number in the grade designations generally indicate the nominal Brinell hardness values of the steel at the surface. These steels are produced to meet specific nominal surface hardness requirements and not to conform to any tensile requirements. There is however a correlation of hardness and tensile strength as set out below:
Bending ROQ-last TH400 and ROQ-last TH360 may be readily formed, provided the following precautions are met:
Machining Machining can be performed using high-speed tool steels and reducing cutting speeds to 50% of speeds used for normal carbon steels. Welding ROQ-last is readily weldable. Low hydrogen procedures must be used to avoid cracking in the heat affected zone. In addition, low tensile strength consumables should be employed to minimise residual stresses. High heating inputs will reduce the hardness in the heat affected zone and must be avoided, especially in multi pass welding. Preheating Temperatures The objective of pre-heating is to ensure a good microstructure in the heat-affected zone (HAZ) by reducing the cooling rate. An excessive cooling rate is caused by:
In such a case a brittle martensitic microstructure, which is susceptible to hydrogen cracking, could arise in the HAZ. Increasing the temperature of the parent plate by pre-heating is usually the easiest way to overcome the problem. The recommended values are given below. They are based on minimum heat-input rates, and preheating temperatures should be increased at lower rates of heat-input.
Minimum heat-input values
Hydrogen-level control Low hydrogen welding processes are widely used. These processes entail the selection of low-hydrogen consumables, and ensuring that the manual metal-arc fluxes are thoroughly dried in accordance with the manufacturer's specifications. The plates must be free from moisture, oil or grease before welding commences. Reduction of hardness in the Heat-Affected Zone (HAZ) The desired properties of ROQ-last plates are achieved by water quenching and tempering at temperatures up to 425deg C. Heating above the tempering temperature, such as during welding, will inevitably reduce the hardness of the plate. Refer to the test certificate for the actual tempering temperature when drawing up welding procedures. Limit Heat-input It is not possible to avoid a certain amount of hardness reduction in the HAZ during welding, but this can be minimsed by limiting the heat-input to a predetermined maximum value. If for instance, a drop in hardness to a value of about 270 Brinell hardness (28 Rockwell C) can be tolerated, the maximum heat-input values, indicated in the following table, could be used. These values can be exceeded if lower hardness values in the HAZ are acceptable e.g. in hidden corners
WELD PROCEDURE TEST Before any critical welds are made, it is advisable to conduct a weld procedure to check the hardness profile across weldment. RESIDUAL STRESS LEVEL The residual stress in a weldment is determined mainly by:
External Restraint Since the method of installing wear-resistant plates usually induces a high level of external restraint, particular attention must be given to fit-up and yield strength of the weld metal. Fit-up Fit-up is important, especially important, especially in small or single run welds, where the aim should be to keep the root gap below 0.4mm. Yield strength of the Weld Metal The selection of the correct filler metal is essential to restrict the residual stress in the weldments. During cooling, either the parent plate or the weld metal has to yield to accomodate the shrinkage stresses. Since the parent plate has a very high yield strength, it is imperative to use a filler metal with a low yield strength to prevent the generation of excessive residual stresses. If it is considered essential to provide for abrasion resistance of the weld bead itself, it is advisable to initially deposit "soft" beads and to apply wear-resistant beads on the surface. SHEARING AND PUNCHING ROQ-last plates can be shreared in thickness up to 25mm. Because of the high hardness of ROQ-last compared to structural steel, the machine capacity should be decreased to 40% of the normal capacity. Punching of ROQ-last is not recommended. GAS CUTTING Owing to the good weldability of ROQ-last grades, the steel can be readily cut by oxy-acetylene flame. Some distortion could be experienced as cuts are made because the low tempering temperature of the steel does not relieve all the internal stresses induced during the quenching process. If subsequent bending is to be done, grinding of the flamecut edges is recommended. ROQ-last can be successfully cut by plasma, laser and hydro jet cutting techniques. HOT - WORKING ROQ-last is normally tempered at temperatures below 425deg C. To ensure that full hardness is retained, the steel should not be hot- worked at temperatures above 330deg C. Hot working should be done at temperatures at least 30deg C lower than the actual tempering temperature. The tempering temperatures are indicated on the test certificate. Hot working of ROQ-last TH400 is not recommended as it is normally supplied in the quenched condition. EDGE CONDITION Plates thicker than 12mm normally have flamecut edges whereas plates 12mm and thinner normmaly have sheared or plasma cut edges. APPLICATIONS
CERTIFICATION Test cerficates verifying hardness and chemical composition will be supplied with all material.
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