Field Hardness Testing, Is there a Minimum ?

[bmoorthy]

Hello All

This was a Vessel that required PWHT and the requirement was due to service requirement. A limit on Max hardness was given as 200 BHN.

We used an Echotip equipment (Approved by the Customer). The callibrations were OK. The readings were also cross verified with the standard block that was provided with the equipment (Before and after field test) and the variations that was observed in the readings very minimal (In fact the observations were with in 3 units + -)

Now during the field check the hardness were found ridiculously low in some places (No specific pattern observed in the locations of the readings). Say 110 and 123, 97 BHN etc. All the readings were lesser than 180 BHN.

There was a Production test coupon (Only Impact tests were called for) the impact test was satisfactory (60J Average and 56 Jouls Min observed).

The hardness test in the Lab showed 135 Bhn 177, 160Bhn (although Hardness was not specified in the Production test coupon) In the Lab we verified the hardness in the Cross section.

One of our corrosion experts during the discussion mentioned that low hardness also leads to failure in some of the services and quoted NACE Publications.

When mentioned about the field observations, he further elaborated in difference between surface hardness and the cross section hardness.

When i said that if surface hardness was so important i could easily increase by simply tapping the low hardness area and increase the hardness the discussion ended.

Now are ther services that specifies MINIMUM hardness aswell. All along i am under the impression that the high hardness not OK and lower hardness is fine.

can some one give feed back on risks of low hardness and is it possible to specify min hardness objectively in construction requirements.

What should on do if lower hardness is observed in some points in a vessel?

 

[swall]

Is the Echotip hardness tester an indentation hardness tester? If so, the soft readings could be due to decarb on the steel surface, particularily with a low load type of indentation hardness tester,such as a microdur tester.

 

[swall]

Oops. After a second google search I found this tester (you spelled it wrong) and it is some sort of impact/rebound type. So my previous comments would be invalid. Don't have any experience with these.

 

[unclesyd]

What is the material of construction?
If you have is can you give the thermal history?

 

[Texastrav]

How thick is the part under test? Equotip recommends that you develop a conversion for thinner materials by averaging five Etip readings and comparing them to a more reliable indention type tester (TeleBrinell)on similar thickness material. Over the years we have found the Etip to produce very low readings on thinner sections of equipment.

 

[stanweld]

While the Equotip hardness tester has its uses, in my 35 years of experience, field testing of pipe and pressure vessels for use in petrochemical and power generation facilities has not been one that provides consistent, reliable results.

 

[bmoorthy]

The Thickness is 25 mm. Material of Construction is SA 516 Gr 60, SA 105, SA 106 Gr b, SA A 350 LF2, SA 333 Gr 6.

Welded with E 7018-1.

PWHT carried out at 610 Deg C. Soaking Time 1 Hour. Rate of Heating 70 Deg C (Due to furnace limitations). Rate of Cooling 60 Deg C (Due to the same reason)

 

[metengr]

No, for P-No 1 materials, there is no minimum hardness because for typical PWHT and tempering applications you have a minimum tempering temperature to assure adequate mechanical properties.

Codes and Standards permit higher tempering and PWHT temperatures above minimum values stated as long as you do not exceed the lower critical transformation temperature. Exceeding the lower critical transformation temperature would result in higher hardness and adverse effects on ductility or toughness of the material.

Regarding field hardness data; I would only use Equotip hardness testing as a means of a quality control check to assure no hardening due to improper PWHT or tempering (either too low a temperature or having exceeded the lower critical transformation temperature). If you have readings this low, lightly grind the surface to remove any scale and try again. My guess is that the surface may not have been acceptable for the Equotip machine. I have performed many a hardness testing using a portable Equotip machine and surface preparation is first and foremost. In addition, the indenter probe must be perpendicular to the surface, any slight movement or angle will result in low readings.