[MW:2002] Re: effect of H on notch strength

Ali,

The name is Owen, not oven. An oven is something in which you heat food, or occasionally heat-treat metals!

 

For hydrogen embrittlement to occur, you must have monatomic hydrogen diffusing through the metal and gathering in grain boundaries. Monatoms of  hydrogen are small enough to diffuse through grains of steel. The embrittlement occurs as a result of monatomic hydrogen atoms diffusing into the grain boundaries, reacting to form molecules of hydrogen gas, which then force the grains apart. Diffusion and reaction rates both slow with decreasing temperature, hence the effect of hydrogen embrittlement decreases below a certain temperature. The effect of the ductile-brittle transition then becomes more important with decreasing temperature. It is also worth remembering that, at constant pressure, the volume of a given mass of hydrogen will decrease with temperature, or, at constant volume, the pressure will decrease with decreasing temperature, hence the driving force pushing the grains apart will be reduced. If it is too cold for the hydrogen atoms to diffuse through the steel, HE can't occur.

 

The monatomic hydrogen is commonly formed as a result of acid/metal corrosion reactions (e.g. pickling steel prior to painting) or other reactions in which hydrogen is liberated as a cathodic reaction (e.g. fusion welding where water is present or there are hydrogen compounds in the flux). The hydroxonium (or hydrogen) ions are reduced to monatomic hydrogen by electrons from the dissolving (corroding) metal or the welding current flow. The monatomic gas can diffuse through metal grains - the diatomic gas molecules can't.

 

2H⁺ + e^- ® 2H^.

2H^. ® H₂

 

 

Some steels are much more prone to hydrogen embrittlement than others. HSLA and high strength carbon steels tend to be more problematic than others.  Hydrogen embrittlement can be prevented by suitable heating to an appropriate temperature after the process which generated the hydrogen (one of the functions of post-weld heat treatment), to allow the hydrogen to diffuse out of the steel. If you have a properly qualified weld procedure, the PWHT should be designed to mitigate the effects of hydrogen embrittlement.

 

When HE is suspected in cases of failure, there is never any sign of the hydrogen - it has all long gone, so it is usually inferred from the circumstances and the nature of the cracks.

 

Regards,

Owen.

 

----- Original Message -----

From: Ali Asghari

To: materials-welding@googlegroups.com

Sent: Tuesday, April 21, 2009 12:36 PM

Subject: [MW:1997] Re: effect of H on notch strength

hi oven

thank you for answer,but i dont understand yet.

because the toughness in -100 is more than in 50,why?

another question:this figure is true in all steel?

regard advance

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From: Owen Jenkins <owen@osjl.co.uk>
To: materials-welding@googlegroups.com
Sent: Sunday, April 19, 2009 12:20:17 PM
Subject: [MW:1985] Re: effect of H on notch strength

Ali,

The top line shows the toughness of the base material without hydrogen embrittlement occurring - toughness increases steadily with increasing temperature.

 

The lower, V-shaped line shows that the effect of hydrogen embrittlement is worst at about 50°C, decreasing rapidly as the temperature increases (and the ability of the hydrogen to diffuse out of the metal increases) and also decreasing at lower temperatures, where reaction rates are much lower and the density of hydrogen increases, hence its volume decreases.

 

Other may wish to add more to this simple approach. Does this help?

 

Regards,

Owen.

----- Original Message -----

From: Ali Asghari

To: materials-welding@googlegroups.com

Sent: Saturday, April 18, 2009 11:31 AM

Subject: [MW:1980] effect of H on notch strength

hi

can anybody explain about this picture?

rgds.

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