01-09-2016, 02:37 PM
1452147653-18HeatTreatmentofSteels090905.pdf (Size: 67.12 KB / Downloads: 5)
We use heat treatments to change properties of metal, or as a method of controlling
formation of structures, or expansion/contractional forces during welding.
In heat treating metals and alloys there are many elements for the welding inspector to
check that may be of great importance, such as the rate of climb and any hold points in
the heating cycle. The holding or soaking time is generally calculated at 1hour for every
25mm of thickness, but this can vary. Heat treatments that are briefly covered in this
section are as follows:
1)
3)
5)
Annealing
Hardening
Stress relieving
2)
4)
6)
Normalising
Tempering
Pre-heating
The methods/sources that may be used to apply heat to a fabrication may include:
a)
b)
c)
Flame burners/heaters (Propane etc.) Preheating
Electric resistance heating blankets. Pre-heating & PWHT
Furnaces. Annealing. Normalising. Hardening. Tempering
The tools that an inspector may use to measure the temperatures of furnaces and heated
materials may include.
a)
b)
c)
d)
Temperature indicating crayons (Tempil sticks). Pre-heating
Thermo-couples. All heat treatments.
Pyrometers (Optical. Resistance. Radiation.). Furnace heat treatments
Segar cones. Furnace heat treatments
The welding inspector should observe that all heat treatments are carried out as specified
and make records of all parameters. This is a critical part of the duties of a welding
inspector who should also ensure that all documents are retained within the quality files
Annealing is a heat treatment process that may be carried out on steels, and most metals
that have been worked hardened or strengthened by an alloying precipitant, to regain the
softness and ductility. In the latter case we generally refer to solution annealing. In
work hardened non-ferrous metals, annealing is used to re-crystallise work-hardened
grains. When annealing most work hardened non-ferrous alloys the cooling rate is not
always critical, and cooling may be rapid without forming any hardened structures. In
steels we can carry out 2 basic kinds of annealing:
a)
b)
Full Annealing (Including Solution Annealing)
Sub Critical Annealing
In full annealing of steels the steel is heated above its UCT (upper critical temperature)
and allowed to cool very slowly in a furnace. This slow cooling will result in a degree of
grain growth, which produces a soft and ductile structure. There are no temperatures that
can be quoted for annealing steels, as this will depend entirely upon the carbon content of
the steel.
The UCT range of Plain Carbon Steels ranges between 910 – 723C, however the
temperature is mostly taken to 50C above the calculated UCT to allow for any
inaccuracies in the temperature measuring device. Plain carbon steel of carbon content of
0.2% would have an annealing temperature in the region of 850 - 950C
The solution annealing of some metallic alloys may require a rapid cooling rate.
In sub critical annealing the steel is heated to temperatures well below the lower critical
temperature (723C). This type of annealing is similar to that used with non-ferrous
metals as it is only the deformed ferritic grains that can be re-crystallised at these lower
temperatures.
The term annealing generally means to bring a metal, or alloy, to its softest and most
ductile natural condition. In steels this also means a reduction in toughness, as the
resultant large grain structure shows very low impact strength.