Hardness Testing Before & After PWHT

It depends (particularly on the clients requirements), but for generic examples:

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API 934-C Materials and Fabrication of 1-1/4Cr - 1/2Mo Steel Heavy Wall Vessels ...

7.4.2 Hardness of Weld Deposit and Adjacent Base Metal

7.4.2.1 After final PWHT (see 7.6) hardness determinations should be made for each pressure-retaining weld (including each nozzle and attachment welds) using a portable hardness tester.

7.4.2.2 Each hardness test result should be the average of three impressions at each test location. The test locations should include weld metal and base metals adjacent to the fusion line on both sides. All individual hardness values should be reported.

7.4.2.3 Hardness values should not exceed 225 HBW.

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NACE MR0175-203 Metals for Sulfide Stress Cracking and SCC Resistance in Sour Oilfield Environments

5.3.1.2 Welding procedures for carbon steels and
low-alloy steels may control welding variables to
achieve a hardness of 22 HRC maximum in the
weldment. The controls generally involve
restricted base and filler metal chemical
composition and welding parameters. The
procedure qualification shall verify that the 22 HRC
maximum hardness requirement is achieved in the
weld deposit, HAZ, and base metal in the aswelded
condition. The resulting welding procedure
specification shall document the required controls
to assure that the 22 HRC maximum hardness
requirement will be achieved in production
weldments.(11)

5.3.1.3 Carbon steel and low-alloy steel
weldments produced without restrictions on base
and filler metal chemical compositions and welding
parameters in accordance with Paragraph 5.3.1.2
shall be post-weld heat treated at a minimum
temperature of 621°C (1,150°F) to produce a
hardness of 22 HRC maximum.

Apart from what our guest has suggested, I am giving below the requirement of hardness tests before and after PWHT.

The term, hardness of a metal can not be precisely defined and has many meanings. In general, hardness usually implies a resistance to deformation and in case of metals, it is as measure of their resistance to plastic or permanent deformation. the harder the metal is, the more it resists scratching, wear, penetration, mechanical cutting etc.

Tensile strength is also increases as hardness increases. hardness of a base mental and weld metal of a welded joint is affected by chemical composition, metallurgical effects of welding, cold working, heat-treatment and several other factors.

Limitations are to be placed on the hardness of Heat affected Zone, and weld metal because if they are too hard, they will not have sufficient ductility (elongation) for the service conditions and their corrosion resistance are also be impaired.

Due to air hardening properties of alloy steels (with Cr & Mo,V) the Weld and HAZ area will build up considerable stress and hardness during welding and is prone to cracking & corrosion during service.

During the PWHT, which is a form of removing the stresses on weld and HAZ, if the heating or cooling rates are not properly controlled, then, the same above conditions will appear.

Hardness checking method is said to be the only way during fabrication, through which one can determine the ductility properties of weld and HAZ.

Out of 30 hardness methods, Rockwell, Brinell and Vickers are widely used.

Trust above details are useful to you.

Sridhar

Thank you very much. Your reply is really very informative.

What about the acceptance criteria? Which code or standard specifies required hardness, say 516 gr 70 before and after PWHT?

Hello

i think that first you've to define the type of service and codes that apply, i mean if you are in a "sour service" H2S service then you shall apply the nace / iso 15156 in upstream or MR 103 in downstream facilities..

what are the fluid, type of service and operating condition?

Vitt

corrosion & rust control

You can follow ASME Sec. VIII for general requirements towards pre- post stress relieving requirements or any specific codes under BS or DIN etc. You can check up these factors in ASTM standards also.

The Pre heat and PWHT are meant for low alloy steels with Cr, Mo, NI, V, Cu, Zn, alloy additions (or) Mild carbon steels like 516 gr. 70 (which are normalized to give more yield strength up to 690 N/mm² ) with higher thicknesses.

Another reason to PWHT in CS are to provide fine grain structure (disturbed during un even cooling during welding), improve notch toughness and meet the impact test requirements at sub-zero temperatures.

Hence, In carbon steels the hardness will be app 180 prior to welding and should not cross 200-220 BHN, after PWHT at 620⁰ C.

In alloy steels (as mentioned earlier) the weld and HAZ should not form marten site structure (due to un even heat input during welding) but always be in a softer ferritic structure.

The practical method to check this structure is not by analyzing its micro every time but by easy method of checking its hardness at weld and HAZ and it should not shoot up beyond 240 BHN. This also helps us to find the correctness of PWHT Cycle.

The PWHT temperature varies depending upon Cr- Mo and other alloy content of the steels. Example 600-740^oC for 1 1/4 Cr -1/2 Mo (or) less, 680-760^oC for 9Cr -1 Mo or less in composition.

Sridhar.

One correction please.

The practical method to check this structure is not by analyzing its micro every time but by easy method of checking its hardness at weld and HAZ and it should not shoot up beyond 240 BHN. This also helps us to find the correctness of PWHT Cycle.

It should be corrected as:

at weld and HAZ and it should not shoot up beyond 220-225 BHN.

Thank you.

thanks guys. You have been very helpful...

Any <...T...> that takes place without knowledge of where the pass/fail boundary is before it takes place isn't a <...T...>; it is simply a measurement.