A STEEL is generally defined as a mixture of carbon and iron having carbon content ranging from a hundreds of a percent to around 2 wt%. Different mixing elements can amount to about 5 wt% in low-alloy steels and the amount can be higher various other alloyed steels like stainless steels and tool steels. Steels could demonstrate a large array of properties based on composition along with the presence and phases of micro constituents, which is directly dependent on the heat treatment.
STRESS-RELIEF HEAT TREATMENT is utilized to alleviate stresses which are remained locked in a structure because of the sequence of manufacturing. This definition differentiates postweld heat treating from the stress-relief heat treating. The goal of postweld heat treating is to offer some preferred metallurgic properties or structure in addition to the residual and relief stresses. For instance, postweld treatments are given to most ferritic weldments in order to enhance the fracture toughness of the zones which are affected by heat (HAZ). Further, nonferrous alloys and austenitic are oftentimes treated postweld heat improve environmental damage resistance. Stress-relief heat treating implies uniform heating of a portion or structure thereof, to a favorable temperature below the range of transformation, retaining at this temperature for a predefined time-frame, succeeded by homogenous cooling. To guarantee uniform cooling, special care must be taken, especially when a component is combined of sections of different sizes. Without constant and uniform rate of cooling, new residual stresses may result greater than or equal to those intended to relieve by a heat-treating process. High stresses and distortion from welding process may be reduced by stress-relief heat treating which can affect working performance. Stress-corrosion cracking (SCC) may occur in the area of components which have remained cold strained at the time of processing. additionally, cold strain per se can generate a reduction in elevated temperatures and creep strength. Ferritic steel with residual stresses may cause important reduction in brittle fracture resistance. Brittle fracture immune material like an austenitic stainless steel, residual stresses would be enough to offer the stress essential to promote SCC even in benign conditions.
NORMALIZING OF STEEL is a process of heat-treating which is generally considered as both microstructural and thermal standpoints. Normalizing is a cycle of austenitizing heating in the thermal sense followed by cooling in slightly agitated or still air. Atypically, the work is heated to a temperature of about 100 °F or 55 °C above the upper iron carbide phase diagram critical line. The heating portion of the process must generate a homogeneous austenitic phase (crystal structure or face-centered cubic) prior to cooling, in order to properly classified as a normalizing treatment.
A wide array of ferrous products may be normalized. All of the standard medium-carbon, low-carbon, and high-carbon wrought steels can also be normalized along with various castings. Inside the weld-affected area, different steel weldments are normalized to polish the structure. stainless steels, austenitic steels, and maraging steels either not generally normalized or cannot be normalized. A steel supplier are commonly used to anneal tool steels. Normalization could decrease or increase the hardness and strength of a given steel in a defined product form, depending on the mechanical and thermal history of the application product. In reality, the roles of normalizing may overlap with or could be baffled with those of hardening, annealing, and stress relieving.
Grain-structure refinement, improved machinability, modification and homogenization of residual stresses are some of the reasons for normalization. For the break up or refinement of the dentritic structure, normalization may be done to homogenize castings. It may facilitate a more even effect to consequent hardening. In the same manner, normalization may be helpful in reducing the banded grain structure caused by hot rolling for wrought products. It may also be useful in mixed large or large grain size and small grain owing to the practices of forging.
ANNEALING is a general term referring a treatment which comprises of holding at and heating to a favorable temperature, followed by cooling at an appropriate rate, mainly for the metallic materials softening. basically, in annealing produces, plain carbon steels a ferrite-pearlite micro-structure. Annealing of steels may be done to support machining or cold working, in order to sharpen electrical or mechanical properties, or to promote the stability of dimension. The selection of an annealing treatment which will offer an adequate accumulation of various properties such as minimum expense, often involving a settlement. Terms used to refer different types of annealing utilized to steels are descriptive of the equipment used, the method used, or the material condition after treatment.
In PREHEATING, the base metal is heated before welding. It is either heated fully or just the joint surrounding region is heated to a predetermined temperature, referred as the preheat temperature. The temperature of base metal between the passes of welding, cannot be decreased below the temperature of preheating. To attain so, the rate of cooling is minimized by doing the preheating of the metal. Requirement of heating during welding is not always considered owing to the supply of welding heat to the job. Preheating may generate various beneficial effects. But, in the absence of the working fundamentals, there is a possibility of money wastage or the degradation of the weldment integrity. Four primary reasons to use preheat are:
(1) It minimizes the cooling rate in the base metal and weld metal, generating a more tensile metallurgical structure having greater crack resistance.
(2) The slower cooling rate provides a possibility to any hydrogen which may be present to spread out harmlessly without cracking the material.
(3) It minimizes the shrinkage stresses in the adjacent base metal and weld, which is particularly important in extremely reserved joints
(4) It increases some steels over the temperature at which breakable fracture would occur during fabrication.
In addition, preheat may be utilized to ensure specialized mechanical properties, like toughness of the notch. The following range of factors may be considered in determining whether to preheat or not:
Base metal chemistry
Filler metal hydrogen content
Previous cracking problems
In case of following a welding code, the code itself will define the minimum preheat temperature for a given welding process, base metal and section thickness. Attainment of this minimum value must be done irrespective of the variation or restraint in the chemistry of base metal; however, increase in the minimum value could be done if required. While having no welding codes, requirement of preheating must be determined. Id the preheating is applicable, what preheat temperature will be the best suitable. In common, less than 25mm (1 inch) low carbon stainless steel usually not require any preheating. However, according to chemistry, section or restraint thickness increases, diffusible weld metal hydrogen level, the requirement for preheat also increases. There are different methods to find out the needed preheat temperature for a given section thickness and base metal.