HYDROGEN CRACKING
Ø INTRODUCTION:
q It is a one type of cold cracking and also called hydrogen induced cracks.
q Hydrogen cracking occur due to metallurgical problem during welding of low alloy steel, C-Mn steel at ambient temperature.
q When hydrogen presents and microstructure embrittled and also stress presents in steels can occur hydrogen cracking.
q Hydrogen cracks occur after completion of welding. But they may be delayed for a period of 2 days, 1 months, 1 years, or even longer.
q Hydrogen cracks occurs in HAZ and also weld metal. But more susceptibility in HAZ because of high cooling rate and so increases hardness.
Ø PHENOMENA OF HYDROGEN CRACKING:
q During welding of steel hydrogen dissolve in molten weld pool.
q Molten steel has high solubility of hydrogen. It falls as hydrogen liquid steel solidifies and sharply falls as steel cools.
q The equilibrium solubility depends upon the concentration or partial pressure above the molten steel.
q Dissolved hydrogen diffuse in steel at different rate vary with temperature.
q Diffuse hydrogen get together and become hydrogen gas.
q Diffusion of hydrogen is continuous in steel from 60°c to 500°c. As temperature increase diffusion rate also increase
q This hydrogen moves in between lattice of steel very slow speed and interfere with dislocations.
q Due to interfere the atomic lattice by hydrogen which allows plastic deformation.
q When steel has plastic deformed and embrittles structure and stresses present , hydrogen force in that area and there initiate the crack.
q Diffuse hydrogen also escape from steel on cooling of steel.
q Due to slow movement of hydrogen, time is needed for hydrogen to diffuse to high stress concentration and move to the crack tip as crack slowly grows.
q This type of crack occur in HAZ or weld metal and both.
q In brief three factor are necessary for occur hydrogen cracking
1. Amount of supersaturated hydrogen present.
2. Level of tensile stress.
3. Fracture toughness of the weld metal in the welded joint.
q Factors promoting cold cracking:
1. Joint restraint and high thermal severity.
2. Hydrogen in the weld metal.
3. Presence of impurities.
4. Embrittlement of the heat-affected zone is low-allow steels.
5. Weld of insufficient sectional area.
Ø HYDROGEN ABSORPTION DURING WELDING:
During welding the metal is subjected to rapid heating and cooling thus allowing very little time for hydrogen which might have been absorbed by the molten metal to diffuse out.
Several factors influence the amount of hydrogen absorbed during welding is given below.
1. Welding consumables
2. Welding process
3. Welding procedure.
Ø SOURCE OF HYDROGEN INTRODUCING INTO WELD METAL:
1. Moisture in the electrode coating or moisture in the flux.
2. Moisture in the base metal due to rusty, dusty, painted surface.
3. Contamination in the base metal at the joint area like dirt, grease, moisture.
4. Outside atmosphere.
For prevention of hydrogen cracking preheating and post heating is to be carried out at area of welded.
POST WELD HEAT TREATMENT
Ø Introduction:
q Post weld heat treatment is carried out after completed welding to remove the residual stress developed due to welding, rolling, machining.
So remove this type of residual stress post weld heat treatment is carried out.
q Propane, natural gases or electricity may provide the energy of post weld heat treatment. The heat may be applied locally / the welded component may be completely enclosed in a furnace.
q Typical post weld heat treatment used in Fabrication industries are:
1. Stress relieving.
2. Annealing.
3. Quenching.
4. Hardening.
5. Normalizing.
¨ Purpose of the PWHT:
q To relieve internal stresses during welding, rolling, machining etc.
q To improve mechanical properties of material.
q To change material grain size.
Ø Applicable Code Specification:
Applicable code is ASME SEC VIII DIV – I for selection of PWHT thickness temperature range and time period.
¨ UCS – 56 for carbon and low alloy steels.
¨ UHA – 32 for high alloy steel.
¨ UNF – 56 for non-ferrous metal.
¨ UCL – 34 for clad vessel.
Post weld heat treatment is mandatory under the following condition:
1) For welded joints over 1.5 inches nominal thickness.
2) For welded joints over thickness 1.25 unless preheat is applied at a minimum temp. of 200 F during welding.
PREHEATING
Ø Definition:
Preheat is the temperature immediately prior to the commencement of welding resulting from the heating of the parent metal in the region of the weld.
Normally, a maximum preheat temperature is required when it is necessary to achieve particular levels of toughness and /or strength in the HAZ or weld metal a maximum value may also be required preheat may be local.
¨ Requirement of PREHEAT:
1. The use of preheat lowers the cooling rates in the weld metal and in the heat affected zone(HAZ). These results in amore ductile metallurgical structure, that will reduce the chances of cold cracking.
2. The slower cooling rate permits hydrogen to diffuse out harmlessly, without causing cold cracking.
3. Preheat reduces shrinkage stress in the weld and adjacent base metal.
4. It also brings some steel above the temperature where brittle fracture might occur during welding.
5. It reduces the amperage required for welding.
6. Improves penetration.
¨ Factors affecting selection of preheating temperature:
1. Type of material
2. Chemistry of material
3. Carbon equivalent of material
4. Ultimate tensile stress(UTS) / Mechanical properties
5. Thickness of the base metal to be welded
6. Restraint condition.
¨ Empirical formula for finding preheat temperature of ferritic steel:
To calculate the preheat temperature of low alloy steel and carbon manganese steel the following formula is used
Preheat temperature (°C) T = 350 Ö CE (1+ 0.5t) – 0.25
Where. t = Thickness of base metal
CE = % C + %Mn + [ %Cr + %Mo + % V] + [%Ni + %Cu]
6 5 15
Width of preheating material is 3 times the plate thickness on each side at opposite side of welding to minimum of 120mm width.
AS per ASME SEC VIII DIV – I Annexture the following preheating temperature is recommended:
| Material | Preheat temperature |
| 1.P.No.1 Gr. No.1,2,3 | q 175 °F(79°c) for material which has both maximum carbon content in excess of 0.30% specified thickness at the joint in excess 25mm. q 50°F(10°c) for all other material in this P-number |
| 2.P.No.3 Gr. No.1,2.3 | q 175°F(79°c) for material which has either a specified min. tensile strength in excess of 70,000 psi or 480 Mpa or a thickness at the joint in excess of 16mm q 50°F(79°c) for all other material in this P-number |
| 3. P.No.4 Gr.No.1,2,3 | q 250°F(121°c) for material which has either a specified minimum tensile strength 60,000psi (410Mpa) or a thickness at the joint in excess of 13mm q 50°F(10°c) for all other material in this P-number |
| 4.P.No.5A &5B Gr.No.1 | q 400°F(204°c) for material which has either a specified minimum tensile strength in excess of 60,000 psi(410 Mpa) of has both a specified minimum chromium content above 60% and a thickness at the joint in excess of ½ in (13mm) q 300°F (149°c) for all other material in these P-number. |
| 5.P.No.6 | q 400°F(204°c) |
| 6. P.No.9 | q 250°F(121°c) for P.No.9A Gr.1 q 300°F |
| 7. P.No. 10 | q 175°F(79°c) for P.N.10A Gr.1 q 250°F(121°c) for P.No.10B Gr.2 q 175°F(79°c) for P.N.10A Gr.3 q 250°F(121°c) for P.No.10F Gr.6 |
| For P.No. 10D Gr.No.-4 & P.No. 10E Gr.No.-5 materials 300°F(149°c) within Interpass temperature maintained between 350°F & 450°F(177°c & 232°c) | |
POSTHEATING
Ø Definition:
Post heat is a heat that is applied immediately after welding
The application of heat to a weld immediately after welding and before cooling out, so as to maintain interpass temperature to raise it to same higher value (well below any PWHT temp.)in order to increases the rate of diffusion of hydrogen out of the weldment.
q As we do post heating the weld metals cools have a relatively slower cooling rate, which give following beneficial effects.
1. Reduce susceptibility to formation of hard structures.
2. Diffusion of dissolved hydrogen from the weld metals.
q Post heating of weld metals is required especially when welding highly hardnable steels, low alloy steel to reduce the cooling rate and prevent cold cracking.
q Its purpose is to effect diffusion of hydrogen from the joint and reduce susceptibility to the associated form of cracking.
q It is usually applied to the higher strength carbon manganese steels and the low alloy steel where the risk of hydrogen induced cracking is higher.
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