HAZ i.e. Heat Affected Zone of weld joint is the area adjacent to weld joint undergoing fast heating and cooling cycles affecting the structure and property of parent material. The welding & QC engineer need to determine whether the changes taken place in the HAZ are detrimental or not. If found detrimental these need to be corrected by control methods. Weldability of steel becomes difficult as the carbon content and alloying elements go on increasing. Welding on a hardened part makes it more difficult for welding and prone to crack in welding and HAZ. Hence it is necessary to quantify detrimental effects of HAZ and effective control of the same. This helps to avoid failures of the part when put in service. The subject paper gives detailed guidance how to verify the HAZ for its ill effects, how to control the HAZ by correct adoption of heat treatment and its acceptance norms for steels.
Summary:
We are aware that welding Heat Affected Zone (HAZ) of weld joint can have detrimental effects on properties and the component can fail if the effects are not evaluated and controlled. Unlike the Pressure Vessel and Special Projects fabrication sector, there is poor awareness in automobile fabrication for guidance to decide acceptance criteria and control methods. The fabricators/ suppliers do not have reference guidelines what should be followed. HAZ, if not taken care can cause failure of parts in service due to high hardness. The subject paper gives detailed guidance how to evaluate the welded steel joints & HAZ, changes taking place in HAZ, how to control the HAZ by correct adoption of heat treatment and its acceptance norms. The subject paper will help understand the significance of HAZ of welding. It will help how to measure severity of HAZ for as-welded part and compare against its acceptance criteria given in paper. If the as-welded part is not meeting the acceptance criteria, the fabricator shall adopt the control methods i.e. addition of preheat/ post-heat/stress relieving operation as mentioned in the paper.
HAZ i.e. Heat Affected Zone of welding:
HAZ i.e. Heat Affected Zone of welding is a part and parcel of welding joint. It is adjacent area to weld joint and undergoes fast heating and cooling cycles during welding. This affects the structure and properties of parent material being welded. HAZ is inevitable and cannot be avoided in welding. However, one needs to evaluate the detrimental effects if any in the welding joint and the affected zone near to weld joint.
Fig.1: Macro graph of a typical fillet weld joint.
Factors affecting HAZ in steel welded joints:
HAZ in steel welded joints will depend on following:
A. Chemical composition of steel being welded
B. Thickness and mass of the part directly affecting the heating/ cooling rate of the part during welding and subsequent cooling.
C. Hardness of steel being welded
D. Prior working of steel i.e. cold working such as drawing, bending, forming, etc.
E. Heat treatment condition of steel
Fig.2: Typical examples of failures in HAZ.
What if Weld Joint HAZ property not controlled:
If hardness is not controlled in HAZ, it may result into crack initiating from HAZ. The high hardness points act as stress raisers making it highly susceptible for crack initation. The martensite structure in HAZ of weld joint remains untempered hence sensitive to stress application. The martensite structure also absorbs hydrogen in subsequent processes like plating where acid baths are involved. This leads into hydrogen embrittlement.
Measurement of HAZ hardness:
Hardness test is an effective and widely used measure for severity of HAZ. Macro test is carried out to measure hardness across weld joint, parent metal and heat affected zone of the weld joint.
Vickers Micro hardness test with 1 kg/ 3 kg load shall be performed by metallurgy lab in accordance with ISO 9015-1.
For weld thickness less than or equal to 5 mm, only one row of indentation traverse shall be performed at a depth of up to 2 mm below the upper surface of the welded joint.
For weld thicknesses above 5 mm, one row of indentation shall be made from each side at a depth of up to 2 mm below the upper surface of the welded joint.
For double sided weld joints, one additional row of indentation shall be measured at root portion of weld joint.
For each row of indentation minimum three hardness readings shall be measured in each of the following areas:
• the weld region
• both heat affected zones of both sides
• both side parent metals
For the HAZ, the first indentation shall be placed as close to the fusion line as possible.
The hardness measurements shall be increased observing the HAZ under optical microscope at around 100 - 500X magnification so that entire width/ spread of heat affected zones are covered.
Few examples to measure the hardness across weld joint in macro test as shown below a) to d)
Fig. 3 for hardness reading points.
Acceptance criteria for as welded components:
The results from the hardness tests as measured in section 4 shall meet the requirements given in Table 1.
Table1: Maximum Permitted hardness values (HV1) for as-welded parts
If the hardness measured is more than the permitted values shown in table 1, then we need to employ additional control methods in welding to bring down the hardness to permitted level as given in Table 3.
Evaluate microstructure of HAZ:
The HAZ needs to be studied for presence and extent of martensite. The hard and brittle phase martensite is not desirable. The martensite phase formation will depend on material chemistry. It increases as the amount of % carbon and other alloying elements such as Cr, Mn, Mo, V which directly increases. The other factors which increase martensite structure are thickness and prior working/ treatment on the part being welded. Thicker the part, higher is the cooling rate increasing martensite formation and heavily cold worked or prior heat treated/ hardened part will result into further martensite formation.
Fig.4 Typical zones in weld joint.
The microstructure has to be combined evaluated with hardness readings in the previous sections discussed. However following guideline may be used: -
• Heavy martensite structure in untempered state near weld joint not desired
• Precipitation of carbides and segregation of impurities not desired.
Fig.5 Untempered & Tempered martensite.
Untempered martensite along with hardness above safe limits as discussed in previous can result into Crack initiation from HAZ and or Hydrogen embrittlement.
Control methods for HAZ:
Following are the various heat treatment operations employed for welding to control the HAZ of weld joint.
a) Pre-heat method
b) Post-heat method
c) Post weld stress relieving.
The methods are to be carefully selected because these methods add to operations and significantly increase time and cost.
a) Pre-heat method: -
Preheating employs heating of weld joint fit-up and surrounding area up to 25 mm up to 100 – 200 °C. Heating can be done by oxy-acetylene gas flame set-up or LPG gas heating burner. Since this is before welding hence the name pre-heat.
Preheat allows the welded joint to cool slowly thus controls the structure and hardness in heat affected zone. It reduces harmful martensite content in heat affected zone.
b) Post-heat method/ De-Hydrogenation: -
Post heating employs heating of welded joints immediately after the welding. The weld joint and surrounding area up to 25 mm up to 200 – 400 °C.
Post heating helps removing hydrogen trapped in weld and HAZ zone of materials which are prone to form martensite such as medium carbon steels, alloy steels and hardened parts being welded. Post heating is therefore called de-hydrogenation treatment and prevents crack due to delayed cracking due to hydrogen/ moisture in hard zones. Moisture is usually picked from atmosphere, dust, oil, moisture and flux coating on the welding wires.
c) Post weld stress relieving/ PWHT: -
Post weld stress relieving heat treatment is a stress relieving method generally performed on the entire part or assembly after the welding operation is completed.
This is generally performed on big assemblies, structures and weld thicknesses over 13 mm and heavy welding. PWHT i.e. Post weld heat treatment relieves the stresses formed in welding without affecting the microstructure and properties of the materials.
PWHT is never crossed above lower critical temperature of steel i.e. 723 °C to ensure no metallurgical structure change takes place. Generally 300 – 650 °C temperature range is used for PWHT considering the amount of stress relieving desired. In case of hardened & tempered parts, the PWHT temperature shall be same as tempering temperature as both have are used for same purpose i.e. stress relieving.
Carbon Equivalence, CE and Weldability of steel material:
Carbon equivalence is the term used in reference with weldability of steels. Weldability is the ease with which welding is carried out on the material. Lower the carbon equivalence of material, higher is the weldability. As the carbon equivalence of the steel increases with the content of carbon and other alloying elements, the weldability decreases sharply.
As the weldability decreases the control methods such as pre-heat, post-heat and PWHT become extremely important to avoid complexities in welding and failure/ crack propogation in weld and HAZ zone.
The formula for calculating carbon equivalence, CE of steel as per AWS (American Welding Society) is as below:-
Where,
C is carbon, Mn is Manganese, Cr is Chromium, Mo is Molybdenum, V is Vanadium, Cu is Copper and Ni is Nickel content in steel.
Guidance for selection of control methods:
Table 2:- Guidance for selection of control methods if as welded hardness is exceeding permitted values in Table 1
Acceptance criteria for heat treated welded components:
The results from the hardness tests as measured in section 4 shall meet the requirements given in Table 3
Table 3: Maximum Permitted hardness values (HV1) for parts
Pre-heat, Post-heat and PWHT controls shall be suitably tried with trial and error method until we get the maximum permitted.
Refer details in annexure 1 for steel groups mentioned in Table 1 & Table 3
Conclusion: -
a. The steel weld joints and HAZ region shall be evaluated by micro hardness test and micro structure evaluation.
b. If the microstructure and hardness of as-welded joint is exceeding the acceptance as discussed above in the subject paper, additional control methods of Pre-Heat/ Post-Heat/Stress Relieving are necessary.
c. Properly controlled HAZ helps avoid crack in weld joint and pre-mature failure from weld joints when parts put in service.
Refer Annexure 1 on page no. 13 & 14 for steel groups.
Annexure 1:- Grouping system for steels, ISO TR 15608
References:-
a. EN ISO 3834- 2 Comprehensive quality requirements for welding.
b. AWS handbook Volume 1-Welding technology.
c. EN ISO 3834- 2 Comprehensive welding quality requirements
d. ASME Sec IX:- Qualification of welding; Boiler & Pressure Vessel Code
e. ISO 15614- Specification and Qualification of welding procedures for metallic materials- welding procedure test for Steels and Nickel alloys.
f. ISO TR 15608:- Grouping system of metals
g. ASME Sec VIII:- Rules for Construction of Pressure Vessels.
h. ASM handbook Volume 6 -Welding & Brazing.
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