Normalization and tempering heat treatment
Alloy steels and low alloy steels are used in the construction of items and components subject, in operation, to high mechanical stresses, in operation at high temperature. The presence in these materials of alloy percentages such as molybdenum (0.5-1.15%) increases the heat resistance whereas Chromium (0.5% - 10%) gives greater resistance to oxidation at high temperature.
For pressure vessels, for this type of use, the following steels are usually used, defined by ASTM and ASME Standards or equivalent Standards and state of supply (sheets - tubes – forged materials):
- ASTM / ASME A / SA832-22V with composition 2¼Cr -1Mo ¼ V, used for high thicknesses, with particular hardness requirements, high toughness and heat resistance;
- ASTM / ASME A387 Grade 9 -91 Class 2 – composition 7.9-9.6 % of Chromium – 0.85-1 % of Molybdenum, with high resistance characteristics to Creep which make these steels optimal for energy production plants and good resistance to the acid attack.
Normalizing aims to create metallurgical structures with fine and homogeneous grains that cannot be obtained during the casting or which are modified during the operations of forging and the work processes for hot and cold plastic deformation. In this treatment, the cooling rate is essential to obtain optimal metallurgical structures for the subsequent tempering treatment; in relation to the thicknesses of the items and the required final mechanical characteristics, Trater may adopt diversified cooling systems, from the most drastic (quench in water, spray quenching or forced air) to softer ones (cooling in still air or slow in the oven). Unfortunately, when coolings are rapid, deformations of the items cannot be excluded. The cooling rate, as said, is essential for the mechanical characteristics that can be obtained through the subsequent tempering treatment. For very massive items with high thickness, the cooling to the core of the material will be slower than the surface and, inevitably, there will be different metallurgical situations with different mechanical characteristics. For special cases or on specific requests, Trater is able to provide in advance indications on the behavior of steel in the thickness of the item, during the treatment study phase, by using FEM simulations and applying correct heat exchange parameters and specific curves of the material.
Tempering follows normalizing and is useful to distribute and rediffuse carbon in the crystal structures of the materials. Since the presence of residual austenite is probable after normalizing on particularly massive items, for some steels it is necessary to perform the tempering immediately after normalizing, before that the spontaneous formation of martensite occurs with the possible generation of cracks in the presence of strong residual stress states, generated in the component during the normalizing treatment. For steels used in the construction of pressure vessels, the minimum tempering temperature must be defined according to the reference Codes and to the requirements connected to the welding processes’ qualifications, including the PWHTs. A tempering performed at low temperatures, to be applied following an incorrect cooling of the manufactured item during normalizing, can determine the adoption of temperatures insufficient to obtain the correct final tempering of the zones out of balance (HAZ) of the welded item with related high hardnesses.
PWHT (Post Weld Heat Treatment) is a treatment to be performed on welded structures with the aim to temper the zones of the welds with the highest hardness through microstructural transformation of diffusion and to improve toughness and ductility, obtaining at the same time a reduction in the hydrogen content. At the normally adopted temperatures, included between 630°C and 750°C, a strong decrease of the residual stress state (relieving) occurs, with an improvement of the material’s resistance to phenomena of stress corrosion cracking, when existing in operation. The soaking time of the thermal cycle must be the minimum one in order to get the maximum benefit in the welds, whereas the temperature must be lower than the one at which the base material has been previously tempered. For chromium-molybdenum-niobium-vanadium steels and for Grade 9 and 91 steels, the soaking time and, above all, the PWHT temperature are important factors; a few degrees of difference from the prescribed range can lead to considerable problems to the material. Also in this case, as for all treatments, it is essential that the execution of the work is entrusted to competent companies; Trater can meet all of customer’s requests, providing adequate ovens, equipment and knowledge needed to perform these thermal cycles.
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Heat treatments on steels with API qualification - Quenching and tempering treatments
It is a treatment which involves two distinct phases: a first cycle (Quenching) in which the austenisation temperature of the steel is reached followed by a rapid cooling performed, generally, by immersing the product in water and a subsequent tempering cycle adopted to obtain a metal with characteristics with the right compromise between Rm, Re and A%. The temperatures, the cooling speed in the quenching phase and the soaking times are the essential parameters of this treatment as the induced microstructural transformations are all thermally activated and all based on transformation and diffusive processes.
Control of the treatments’ temperature
Trater has developed very valid and efficient methods for controlling and measuring the temperature in all the most critical parts of the items using an adequate number of calibrated thermocouples applied in direct contact with the items.
For the quenching cycle, sensors made of thermocouple wire with ceramic fiber insulation, for high temperatures, are used.
Furthermore, with our systems it is possible to record the exact temperature of the item at the moment of immersion in water, different from that at the moment of the oven opening, due to the inevitable thermal exchanges that occur through radiation and convection with the environment .
Treatment ovens and cooling speed
Trater has:
During quenching in water, in relation to the geometry of the products (exchange surface) and, above all on the basis of the thickness, different cooling speeds of the metal will inevitably be determined between the surface of the item and the core of the material with relative final mechanical characteristics.
Trater is able, in advance, to provide the Customer, through finite element analysis, with a valid simulation of the cooling and the consequent mechanical characteristics obtainable in the thickness.
Deformations
Instead, it is not possible to intervene on the deformations due to the rapid cooling of the items in water due to the inevitable temperature differences that arise during this treatment phase.
Control and recording of thermal cycles – final certification
