A Discussion on Hydrogen Bake-Outs

Introduction

This is the second of two articles published in Inspectioneering Journal discussing the value of hydrogen bake-outs. Our first article was published in the May/June 2013 issue and received a great response from the Inspectioneering community. In this piece, we will continue the discussion and touch on some new technologies used to enhance the bake-out process.

Managing the Bake-out

In the refining and process industries a bake-out is used to drive hydrogen out of steel, as trapped hydrogen can cause cracking in a weld. Typically, the time to perform a hydrogen bake-out happens during a maintenance turnaround, prior to welding, and as such, the need for the bake-out is frequently challenged by an effort to save time and money. The time and temperature needed to bake out the hydrogen to an acceptable level for successful weld repair is widely disputed, and when discussing with metallurgists or corrosion engineers, you will frequently find varying degrees of opinion ranging from none to some. There are rules-of-thumb for “in-house” delay times prior to weld inspection, e.g. between 16 to 48 hours in various standards, but there is no firm basis for these times. There is good reason for delaying the weld inspection after welding, as delayed hydrogen cracking can occur. Furthermore, there is generally no discrimination between different materials, joint geometries, or welding conditions. Due to a lack of detailed knowledge and for simplicity, most adopt a single delay time for all circumstances.

In a hydrogen bake-out, the user heats the steel to an elevated temperature and allows time for the hydrogen to diffuse out of the steel, leaving it hydrogen free and weldable. Several key issues play a role in the success of the hydrogen bake-out, including:

1. Bake-out temperature;
2. The amount of hydrogen trapped in the steel;
3. The solubility and diffusivity of hydrogen in the alloy;
4. Steel thickness; and
5. The alloy’s susceptibility to hydrogen embrittlement (hardness).

The time and cost to remove cracks from delayed cracking, due to inadequate hydrogen bake-outs, and typically leading to multiple repairs, will always be more than the time and cost of performing the hydrogen bake-outs. And, not every attempt in outgassing meets with success.

It is quite well known that cracking can occur immediately during welding, or there can be some delay between the completion of the weld and the formation of hydrogen cracks, but it is usually within 48 hours. Therefore, if traditional inspection is carried out too soon after welding susceptible material, these cracks may not be detected and as a result, a faulty weld is put into service. On the other hand, excessive delays after welding prior to inspection can seriously impact project planning and delay the return of equipment to service.