Heat Treatments
Before or Post Weld Heat Treatment
Before Welding Heat Treatment is a Normalization treatment that can be performed when the "bending radius" of the raw material is too invasive for the product itself, i.e. when the ratio W.t. / Ø ≥ 10%. While the Post Weld Heat Treatment is normally required when the governing W.t. is ≥ 19mm. Based on the product's type and characteristics it can be supplied within its relevant delivery status as per the following examples:
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Stress Relieving Post Weld Heat Treatment
► 20°C ▲ 120°C/h ► 2,5' x mm.Wt.@ 600°C. ▼ 150°C/h ► 100°C
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Normalizing Before or Post Weld Heat Treatment
► 20°C ▲ 100°C/h ► 1,5' x mm.Wt.@ 910°C. ▼ Calm Air
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Quenching & Tempering Post Weld Heat Treatment
Q ► 20°C ▲ 100°C/h ► 1,5' x mm.Wt.@ 910°C. ▼ H2O
T ► 20°C ▲ 120°C/h ► 1,5' x mm.Wt.@ 630°C. ▼ Calm Air
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Solubilization Post Weld Heat Treatment
► 20°C ▲ 100°C/h ► 1,5' x mm.Wt.@ 1050°C. ▼ H2O
Each treatment must always be performed according to the manufacturer of the raw material instructions so as not to impair the performance, and kept under control by means of Destructive and Non Destructive Examinations. The Stress Relieving of a type 304 or 316 stainless steel can only be done provided that the Carbon content falls in the appropriate area shown in the diagram at the top-right in order to avoid the phenomenon of Chromium Carbides Precipitation which would cancel the stainless properties, hence the need of 304L or 316L, where the L stand for Low Carbon.
Definition
In order to improve its behaviour in view of its final use, to endow particular characteristics to the steel, it is normal practice to subject it to heat treatments. A heat treatment operation, or the succession of multiple operations, during which the steel is subjected to one or more thermal cycles within determined limits, where the most important variables are temperature and time. A thermal cycle normally involves heating to a given temperature, a maintenance for a time at this temperature and finally cooling down to room temperature in different ways in relation to the desired effects. The various cycles of treatment are selected according to the characteristics of hardness, toughness, and processability desired. The treatment cycle is to be fixed not only depending on the type of steel but also the size of the pieces, as well as heating and cooling rates.
During heat treatments the steel can reach its critical points, during heating and cooling phases, a phase change may occurr by reaching the transformation point:
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AC1 ▲ begins ferrite to austenite formation
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AC3 ▲ terminates ferrite to austenite formation
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AR3 ▼ austenite to ferrite transformation
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AR1 ▼ ends austenite transformation into ferrite + cementite
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MS ▼ begins austenite transformation into martensite
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MF ▼ ends austenite transformation into martensite
With reference to the austenite transformation diagrams, the curves are commonly named as follows:
a) TTT curves (Temperature - Time - Transformation) bain S-curves
b) CCT curves (Continuous - Cooling - Transformation)
These curves are characteristics for each type of steel and allow to establish temperature, time and cooling rate to be chosen in order to obtain the needed structures and characteristics.
Figure 1 schematically shows the CCT diagram for continuous cooling of a steel ipoeutettoide to which have been superimposed 3 curves, indicated with the numbers 1, 2, 3, representing three different cooling rates. Please note that in the upper area of the CCT curve is produced the pearlitic transformation, in the intermediate zone is produced the bainitic transformation, while at a temperature of MS starts the formation of martensite. The cooling curves intersect the CCT diagram in points corresponding to the appearance of the different constituents:
- the curve n. 1, relating to a low cooling rate, can be related to a cycle of normalization of a particular size of relatively high;
- the curve n. 2 wherein the cooling rate is greatly increased, is typical of cases of incomplete quenching where the structure is not completely martensitic;
- the curve n. 3 which represents the case in which, for the high cooling rate, the curve does not intersect nor the pearlitic transformation zone, nor that bainitic;
the austenite is stable for up to Ms and from this point starts the progressive transformation into martensite that MF is completed to the point. The minimum speed of cooling that gives rise to the complete hardening (100% martensite) is normally defined: critical speed of hardening. Since the curves of start and end processing are moved to the right to the presence of alloying elements, it follows that the critical speed of hardening will be less high for alloy steels than for carbon steels.
Figure 2 shows a TTT diagram in schematic isothermal transformation in which the vertical axis shows the temperature and on the horizontal time on a logarithmic scale. Four curves have been superimposed on to as many isothermal treatments that we will see. The TTT diagrams have different shape and pattern according to the type of steel: however, all delimit in a more or less net two zones: an upper, of the pearlitic transformation and a lower one, of the bainitic transformation. The dashed curve on the left indicates the beginning of the precipitation of ferrite, the continuous curve to the left indicates the start of pearlitic or bainitic transformation, while the right end thereof. In the lower part of the diagram are marked the lines of Ms and MF, which indicates the temperatures of the beginning and end of the martensitic transformation TTT diagrams, which have been obtained with tests carried out on small samples and under certain operating conditions, require some changes to the their use in industrial practice. They give in general a very precise indication with regard to the temperatures, while for the processing time must be taken into account the effect of mass of the parts and other difficulties, which require to take much longer times than those indicated in the diagrams.
