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Basics of Elastomeric Seal Design

Basics of Elastomeric Seal Design

Wednesday, 20 July 2016  |  Kate Kunkel

Engineers need critical design information when choosing a seal for a particular valve application. This includes:

  • Operating temperature and pressure r...

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Industry Headlines

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IMF Cuts Global Growth Forecast After Brexit Vote

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HIC Testing

materials_q_and_a_graphicQ: I’m working on an order in which the customer’s 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

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

It’s 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 purchaser’s 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 This email address is being protected from spambots. You need JavaScript enabled to view it..


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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