hic testing

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Details Published on Tuesday, 18 January 2011 13:22 Written by Don Bush

MATERIALS Q&A

HIC Testing

Q: Im working on an order in which the customers material specification imposes HIC

testing. What is HIC testing?

A: HIC stands for hydrogen-induced cracking; it is related to hydrogen blistering. NACE/ASTM

G1931 includes the following description of hydrogen blistering:

The formation of subsurface planar cavities, called hydrogen blisters, in a metal resulting from excessive

internal hydrogen pressure. Growth of near-surface blisters in low-strength metals usually results in surface

bulges.

Hydrogen blistering occurs most often in carbon steels in wet H2S environments (i.e., applications in which

water and hydrogen sulfide co-exist). Corrosion in this type of environment tends to charge the steel with

monatomic hydrogen. When the small monatomic hydrogen atoms combine at a discontinuity in the steel, they

form larger diatomic hydrogen (H2), which is then too large to diffuse through the steel. As more and more

monatomic hydrogen atoms combine to form diatomic hydrogen at discontinuities, the pressure in the

discontinuities builds until blisters form.

In formed steels, blistering can result in the formation of planar cracks running along the rolling direction of the

steel and parallel to the surface. Cracks on one plane can link up with cracks on adjacent planes to form steps,

which can eventually reduce the effective wall thickness until the component becomes overstressed and

ruptures.2

This phenomenon has been known by many different names over the years, including stepwise cracking,

hydrogen pressure cracking, blister cracking and hydrogen-induced stepwise cracking. NACE and ASTM have

standardized on the name hydrogen-induced cracking for this phenomenon in NACE/ASTM G193, defining it

as:

Stepwise internal cracks that connect adjacent hydrogen blisters on different planes in the metal, or to the

metal surface.

Both hydrogen blistering and HIC are encountered most often in plate and in rolled and welded pipe made from

plate. These items exhibit a flat, planar grain structure and often contain large, planar sulfide inclusion, which

helps to promote the blistering and cracking mechanism. HIC has also been reported in other forms (welding

fittings, seamless pipe and forgings), although it occurs much less frequently in those material forms.3

A number of methods are used to try to mitigate hydrogen blistering and HIC. First and foremost is the use of

killed steels, i.e., steels that are deoxidized with silicon, aluminum or some other strong, oxide-forming

element to prevent internal porosity in the poured ingot. Porosity in an ingot can remain as internal voids in

finished products, and those internal voids are prime locations for the formation of hydrogen blisters.

The next level of mitigation is the use of so-called clean steels. Clean steels contain very low concentrations of

sulfur (and usually phosphorus). This results in very low concentrations of non-metallic inclusions in the steel,

which can also serve as sites for blister formation.

In addition to reduced sulfur contents, calcium or certain rare earth elements can be added to steels to control

the shape of sulfide inclusions. The resulting spheroidal inclusions provide better resistance to blistering and

HIC than the normal elongated (and flat, in the case of plate) inclusions.

When a customer imposes HIC testing, it usually means that testing must be performed in accordance with

NACE TM02842. This standard outlines:

1. the test solution

2. the testing apparatus

3. the size, shape and location of test specimens

4. the testing procedure

5. evaluation of test specimens, and

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6. reporting of results.

Note that TM0284 does not include acceptance criteria.

Sectioning and preparation of test specimens is rather time consuming. Once the test is in progress, it runs for

96 hours. After completion of the 96-hour exposure, further sectioning is performed on each specimen, followed

by metallographic polishing, etching and examination at 100X magnification. All cracks are then measured for

length and thickness as defined in the test method. In other words, this is a very labor-intensive, expensive test.

Now for the good news. HIC does not occur in castings, regardless of the material. Therefore, it is inapplicable

to cast valve bodies. The current version of TM0284 does not specifically mention piping fittings or forgings, but

the revision currently in progress includes coverage of piping fittings, plate or forged blind flanges, and forged

weld-neck flanges.

HIC does not occur in austenitic and duplex stainless steels, nickel alloys and copper alloys. In addition, none

of the standards relating to HIC mention its occurrence in alloy steels or martensitic stainless steels.

In other words, HIC testing rarely applies to valves. The few exceptions would be:

Butterfly valve bodies made from carbon steel plate

Large, fabricated valves made from carbon steel piping fittings

Carbon steel weld-neck flanges welded to valve bodies

Its up in the air whether flanged bonnets made from carbon steel forgings need to be tested. Forged bonnets

typically have necks much more significant than the necks in a forged weld-neck flange and would exhibit

quite different metallurgical texture than a weld-neck flange. I am not aware of any reported HIC failures in

forged valve bonnets. Note that proposed revision of NACE TM0284 only covers testing of blind flanges and

weld-neck flanges. It does not cover forgings in general.

Many end-users and engineering, procurement and construction contractors systematically group valves under

piping for material selection and specification purposes. Because of this, the requirement to perform HIC

testing on valve materials often occurs as a result of a general piping specification being extended to cover

valves. When this happens, the valve manufacturer needs to bring the situation to the purchasers attention to

avoid unnecessary testing and the associated expense and delivery delays.VM

Don Bush is a principal materials engineer at Emerson Process Management Fisher . Reach him at

Don.Bush@Emerson.com.

References:

1. NACE/ASTM Standard G193-2010, Standard Terminology and Acronyms Relating to Corrosion (Houston,

TX: NACE)

2. NACE Standard TM0284-2003, Evaluation of Pipeline and Pressure Vessel Steels for Resistance to

Hydrogen-Induced Cracking (Houston, TX: NACE)

3. NACE Standard SP0296-2010, Detection, Repair, and Mitigation of Cracking in Refinery Equipment in Wet

H2S Environments (Houston, TX: NACE).

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