8. Strain Age Embrittlement
This phenomenon applies to carbon and low alloy steel. It involves ferrite forming a
compound with nitrogen; iron-nitride (Fe4N). Temperatures around 250°C, will cause
a fine precipitation of this compound to occur. It will tend to pin any dislocations in
the structure that have been created by cold work or plastic deformation.
Strain ageing increases tensile strength but significantly reduces ductility and
toughness.
Modern steels tend to have low nitrogen content, but this is not necessarily true for
welds. Sufficient Nitrogen, approximately 1 to 2 ppm, can be easily picked up from
the atmosphere during welding.
Weld root runs are particularly at risk because of high contraction stresses causing
plastic deformation. This is why impact test specimens taken from the root or first
pass of a weld can give poor results.
Additions of Aluminium can tie up the Nitrogen as Aluminium Nitride, but weld-
cooling rates are too fast for this compound to form successfully. Stress relief at
around 650 degrees C will resolve the problem.
HOW TO AVOID PWHT
The above picture is of a new pressure vessel that failed during its hydraulic
test. The vessel had been stress relieved, but some parts of it did not reach the
required temperature and consequently did not experience adequate
tempering. This coupled with a small hydrogen crack, was sufficient to cause
catastrophic failure under test conditions. It is therefore important when
considering PWHT or its avoidance, to ensure that all possible failure modes
and their consequences are carefully considered before any action is taken.
The post weld heat treatment of welded steel fabrications is normally carried
9. out to reduce the risk of brittle fracture by: -
• Reducing residual Stresses. These stresses are created when a weld
cools and its contraction is restricted by the bulk of the material
surrounding it. Weld distortion occurs when these stresses exceed the
yield point. Finite element modelling of residual stresses is now
possible, so that the complete welding sequence of a joint or repair can
be modelled to predict and minimise these stresses.
• Tempering the weld and HAZ microstructure. The microstructure,
particularly in the HAZ, can be hardened by rapid cooling of the weld.
This is a major problem for low and medium alloy steels containing
chrome and any other constituent that slow the austenite/ferrite
transformation down, as this will result in hardening of the micro
structure, even at slow cooling rates.
The risk of brittle fracture can be assessed by fracture mechanics. Assuming
worst-case scenarios for all the relevant variables. It is then possible to predict
if PWHT is required to make the fabrication safe. However, the analysis
requires accurate measurement of HAZ toughness, which is not easy because
of the HAZ’s small size and varying properties. Some approximation is
possible from impact tests, providing the notch is taken from the point of
lowest toughness.
If PWHT is to be avoided, stress concentration effects such as: - backing bars,
partial penetration welds, and internal defects in the weld and poor surface
profile, should be avoided. Good surface and volumetric NDT is essential.
Preheat may still be required to avoid hydrogen cracking and a post weld
hydrogen release may also be beneficial in this respect (holding the fabrication
at a temperature of around 250C for at least 2 hours, immediately after
welding).
Nickel based consumables can often reduce or remove the need for preheat,
but their effect on the parent metal HAZ will be no different from that created
by any other consumable, except that the HAZ may be slightly narrower.
However, nickel based welds, like most austenitic steels, can make ultrasonic
inspection very difficult.
Further reduction in the risk of brittle fracture can be achieved by refining the
HAZ microstructure using special temper bead welding techniques.
10. Alloying Elements
Manganese
Increases strength and hardness; forms a carbide; increases hardenability; lowers the
transformation temperature range. When in sufficient quantity produces an austenitic
steel; always present in a steel to some extent because it is used as a deoxidiser
Silicon
Strengthens ferrite and raises the transformation temperature temperatures; has a
strong graphitising tendency. Always present to some extent, because it is used with
manganese as a deoxidiser
Chromium
Increases strength and hardness; forms hard and stable carbides. It raises the
transformation temperature significantly when its content exceeds 12%. Increases
hardenability; amounts in excess of 12%, render steel stainless. Good creep strength
at high temperature.
Nickel
Strengthens steel; lowers its transformation temperature range; increases
hardenability, and improves resistance to fatigue. Strong graphite forming tendency;
stabilizes austenite when in sufficient quantity. Creates fine grains and gives good
toughness.
Nickel And Chromium
Used together for austenitic stainless steels; each element counteracts disadvantages
of the other.
Tungsten
Forms hard and stable carbides; raises the transformation temperature range, and
tempering temperatures. Hardened tungsten steels resist tempering up to 6000C
Molybdenum
Strong carbide forming element, and also improves high temperature creep resistance;
reduces temper-brittleness in Ni-Cr steels. Improves corrosion resistance and temper
brittleness.
Vanadium
Strong carbide forming element; has a scavenging action and produces clean,
inclusion free steels. Can cause re-heat cracking when added to chrome molly steels.
Titanium
Strong carbide forming element. Not used on its own, but added as a carbide stabiliser
to some austenitic stainless steels.
Phosphorus
Increases strength and hardnability, reduces ductility and toughness. Increases
machineability and corrosion resistance
Sulphur
Reduces toughness and strength and also weldabilty.
11. Sulphur inclusions, which are normally present, are taken into solution near the fusion
temperature of the weld. On cooling sulphides and remaining sulphur precipitate out
and tend to segregate to the grain boundaries as liquid films, thus weakening them
considerably. Such steel is referred to as burned. Manganese breaks up these films
into globules of maganese sulphide; maganese to sulphur ratio > 20:1, higher carbon
and/or high heat input during welding > 30:1, to reduce extent of burning.
