18.0 HEAT TREATMENT OF STEEL & WELD JOINTS
The heat treatment given to a particular grade of steel by the steelmaker/supplier should be shown on the material test certificate and may be referred to as the ‘supply condition’.
Welding inspectors may need to refer to material test certificates and it is appropriate that they be familiar with the terminology that is used and have some understanding of the principles of some of the most commonly applied heat treatments.
Welded joints may need to be subjected to heat treatment after welding (post-weld heat treatment) and the tasks of monitoring the thermal cycle and checking the heat treatment records are often delegated to welding inspectors.
18.2 HEAT TREATMENT OF STEEL
The main supply conditions for weldable steels are:
as rolled plate is hot rolled to finished size and allowed to air cool;
hot rolled the temperature at which rolling finishes may vary from
hot finished plate to plate and so strength and toughness properties vary and are not optimised;
applied to relatively thin, lower strength C-steel
TMCP* steel plate given precisely controlled thickness
control-rolled reductions during hot rolling within carefully controlled thermo-mechanically rolled temperature ranges; final rolling temperature is also
applied to relatively thin, high strength low alloy steels (HSLA steels) and for some steels with good toughness at low temperatures, e.g., cryogenic steels
* TMCP = Thermo-Mechanical Controlled Processing
normalised after working the steel (rolling or forging) to size, it is heated to ~900°C and then allowed to cool in air to ambient temperature; this optimises strength and toughness and gives uniform properties from item to item for a particular grade of steel;
applied to C-Mn steels and some low alloy steels
quenched after working the steel (rolling or forging) to size, it is
& heated to ~900°C and then cooled as quickly as possible
tempered by quenching in water or oil; after quenching, the steel must be tempered (softened) to improve the ductility of the ‘as-quenched’ steel;
applied to some low alloy steels to give higher strength, or toughness or wear resistance
solution annealed after hot or cold working to size, steel heated to ~1100°C
solution heat treated and rapidly cooled by quenching into water to prevent any carbides or other phases from forming
applied to austenitic stainless steels such as 304 & 316 grades
Figures 1, 2, 3 & 4 are schematics of thermal cycles for the main supply conditions.
18.3 POST WELD HEAT TREATMENT (PWHT)
Post weld heat treatment has to be applied to some welded steels in order to ensure that the properties of the weldment will be suitable for their intended applications.
The temperature at which PWHT is carried out is usually well below the temperature where phase changes can occur (note 1), but high enough to allow residual stresses to be relieved quickly and to soften (temper) any hard regions in the HAZ.
There are major benefits of reducing residual stress and ensuring that the HAZ hardness is not too high for particular steels for particular service applications.
Examples of these benefits are:
To improve the resistance of the joint to brittle fracture
To improve the resistance of the joint to stress corrosion cracking
To enable welded joints to be machined to accurate dimensional tolerances
Because the main reason for (and benefit of) PWHT is to reduce residual stresses, PWHT is often called ‘stress relief’.
Note 1: There are circumstances when a welded joint may need to be normalised to restore HAZ toughness. However, these are relatively rare circumstances and it is necessary to ensure that welding consumables are carefully selected because normalising will significantly reduce weld metal strength
18.4 PWHT THERMAL CYCLE
The Application Standard/Code, will specify when PWHT is required to give benefits #1 or #2 above and also give guidance about the thermal cycle that must be used.
In order to ensure that a PWHT cycle is carried it in accordance with a particular Code, it is essential that a PWHT procedure is prepared and that the following parameters are specified: -
the maximum heating rate
the soak temperature range
the minimum time at the soak temperature (soak time)
the maximum cooling rate
18.4.1 Heating rate
This must be controlled to avoid large temperature differences within the fabricated item. Large differences in temperature (large thermal gradients) will produce large stresses and these may be high enough to cause distortion (or even cracking).
Application Standards usually require control of the maximum heating rate when the temperature of the item is above ~300°C. This is because steels start to show significant loss of strength above this temperature and are more susceptible to distortion if there are large thermal gradients.
The temperature of the fabricated item must be monitored during the thermal cycle and this is done by means of thermocouples attached to the surface at a number of locations representing the thickness range of the item.
By monitoring furnace, and item, temperatures the rate of heating can be controlled to ensure compliance with Code requirements at all positions within the item.
Maximum heating rates specified for C-Mn steel depend on thickness of the item but tend to be in the range ~60 to ~200°C/h.
The soak temperature specified by the Code depends on the type of steel and thus the temperature range required to reduce residual stresses to a low level.
C & C-Mn steels require a soak temperature of ~600°C whereas some low alloy steels (such as Cr-Mo steels used for elevated temperature service) require higher temperatures – typically in the range ~700°C to ~760°C.
Note: Soak temperature is an essential variable for a WPQR. Thus, it is very
important that the it is controlled within the specified limits otherwise it
may be necessary to carry out a new WPQ test to validate the properties of
the item and at worst it may not be fit for purpose.
18.4.3 Soak time
It is necessary to allow time for all the welded joints to experience the specified temperature throughout the full joint thickness.
The temperature is monitored by surface-contact thermocouples and it is the thickest joint of the fabrication that governs the minimum time for temperature equalisation.
Typical specified soak times are 1h per 25mm thickness.
18.4.4 Cooling rate
It is necessary to control the rate of cooling from the PWHT temperature for the same reason that heating rate needs to be controlled – to avoid distortion (or cracking) due to high stresses from thermal gradients.
Codes usually specify controlled cooling to ~300°C. Below this temperature the item can be withdrawn from a furnace and allowed to cool in air because steel is relatively strong and is unlikely to suffer plastic strain by any temperature gradients that may develop.
Figure 5 is a schematic of a typical PWHT thermal cycle.
18.5 HEAT TREATMENT FURNACES
It is important that oil and gas fired furnaces used for PWHT do not allow flame contact with the fabrication as this may induce large thermal gradients.
It is also important to ensure that the fuel (particularly for oil fired furnaces) does not contain high levels of potentially harmful impurities – such as sulphur.
18.6 LOCAL PWHT
For a pipeline or pipe spool it is often necessary to apply post weld heat treatment to individual welds by local application of heat.
For this, a PWHT procedure must specify the previously described parameters for controlling the thermal cycle but it is also necessary to specify the following: -
the width of the heated band (that must be within the soak temperature
the width of the temperature ‘decay’ band (soak temp. to ~300°C)
Other considerations are:
the position of the thermocouples within in the heated band width and the
if the item needs to be supported in a particular way to allow
The commonest method of heating for local PWHT is by means insulated electrical elements (electrical ‘mats’) that are attached to the weld.
Gas fired, radiant, heating elements can also be used.
Figure 6 shows typical ‘control zones’ for localised PWHT of a pipe butt weld.