HEAT TREATMENTS ON
LOW-ALLOY AND ALLOY STEELS
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 (NON CAPISCO).
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 SA387 Grade 12 Class 2 (13CrMo 4-5) - composition 1% of Chromium - 1/2% of Molybdenum - working temperature up to 560 °C
ASTM / ASME SA387 Grade 11 Class 2 - composition 1.25% of Chromium -1/2% of Molybdenum - working temperature effective up to 575 °C
ASTM / ASME SA387 Grade 22 Class 2 - (10CrMo 9-10) - composition 2.25% of Chromium - 1% of Molybdenum - working temperature effective up to 600 °C
ASTM / ASME A387 Grade 5 Class 2 - composition 5% of Chromium - 1/2% Molybdenum
Also the following steels are subject to treatment: SPOSTARE L'ELENCO PUNTATO COME SOPRA
• ASTM / ASME A / SA832-22V with composition 2¼Cr -1Mo ¼ V, used in thick layers, with particular hardness requirements, high toughness and heat resistance;
• ASTM / ASME A387 Grade 9 -91 Class 2 - composition 7.9 to 9.6 % of Chromium - 0.85 to 1 % of Molybdenum, with high resistance characteristics to creep which make these steels optimal for energy production facilities and good resistance to the attack of acid.
On low alloy steels and alloy steels TRATER normally performs normalizing and tempering treatments to obtain the required final mechanical properties and PWHT or stress relieving treatments after welding.
NORMALIZING OF LOW ALLOY AND ALLOY STEELS
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 following tempering treatment; in relation to the thicknesses of the manufactured items and the required final mechanical characteristics, TRATER may adopt diversified (DIFFERENT) cooling systems, from the most drastic (quench in water, spray quenching or forced air) to softer ones (slow cooling in still air or in the oven).
Unfortunately, when coolings are rapid as indicated deformation of the items cannot be excluded.
The cooling rate, as said, is essential to the mechanical characteristics that can be obtained through the following 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 using FEM simulations and applying correct heat exchange parameters and specific curves of the material.
Tempering follows normalizing and is useful to distribute 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 normalization, before the spontaneous formation of martensite, which may cause cracks in the presence of strong residual stress states, generated in the component during the normalization processing.
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, which include also the PWHT.
Low-temperatures-performed tempering, applied following an incorrect cooling of the manufactured item during normalizing, can determine the adoption of insufficient temperatures to obtain the correct final tempering of the zones out of balance (HAZ) of the welded item with related high hardness.
PWHT AND STRESS RELIEVING
PWHT (Post Weld Heat Treatment) is performed on welded structures with the aim to temper the zones of the welds with the highest hardness through diffusion and to improve toughness and ductility, obtaining at the same time a reduction in the hydrogen content.
At the normally used temperatures, included between 630 °C and 750 °C, a strong decrease of the residual stress state (stress 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.