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Techniques for Preventing HICC (Hydrogen Induced Cold Cracking)

We saw in a recent article that there are four items that will have an effect on the probability of a carbon steel producing 鈥渉ydrogen induced cold cracking鈥� in the weld zone. These four things were listed as follows:

1. The temperature 鈥� cracking can only occur after the weld has cooled below approximately 150掳C. i.e., when its relatively cold.

2. The stress level at the point of crack initiation

3. The hydrogen content of the weld zone

4. The susceptibility of the weld and/or of HAZ to hydrogen embrittlement.

In evaluating these four points it鈥檚 safe to say we have absolutely no control on Item 1, if other conditions are apparent, then HICC will occur below a temperature of about 150 deg C (relatively cold). Item 2, the stress level at the point of crack initiation, is affected by such things as the resistance to expansion/contraction of the joint, stress risers at such locations as sharp geometrical weld toes, undercut etc. that are not controlled and, as such, quite difficult to quantify.听

We are, therefore, left with the final two items. These are things we do have control over and we can exercise them by applying sound welding procedural approaches.

鈥淭he Hydrogen Content of the Weld Zone鈥�

Typical hydrogen contents for the common manual and semi-automatic arc welding process are shown in Figure 1. This is measured as ml/100 grams of weld metal. If we regard 16ml/100 grams听 (CH16) of weld metal as the line between low hydrogen and not, then we can say that both GTAW and GMAW can be considered to be low hydrogen welding processes. However, with both the FCAW and SMAW welding processes we have choices of both low or non-low hydrogen welding consumables. It can be readily seen that the hydrogen content is a function of the flux as those welding processes that contain no flux, GTAW and GMAW, are considered low hydrogen. Conversely, fluxed consumables FCAW and SMAW can be either low hydrogen or non-low hydrogen depending on their flux classification.

The commonly used 鈥渂asic fluxed鈥� low hydrogen SMAW electrode, classified in CSA W 48 as an E4918听 has a maximum 鈥渃ontrolled hydrogen鈥� level of CH 16. However, these electrodes can also be classified as much lower hydrogen levels of听 CH 8 and CH 4. The selection of CH 16, CH8 or a CH 4 electrode will normally depend on the strength of the steel being welded. As the strength increases then the steel becomes more susceptible to HICC and a lower CH electrode will be needed.

Figure 1 Typical Hydrogen Contents of Welds Deposited by Various Arc Welding Processes.

The SMAW electrodes classified as low hydrogen (CH) will only produce low hydrogen deposits if they are kept dry and the proper storage conditions are followed. All low hydrogen electrodes have to be controlled as per the following.

• Delivered in hermetically sealed containers

• Stored in holding ovens at recommended temperatures.

Re-baked or discarded after being exposed for a certain period of time.

It is extremely important to note that if low hydrogen electrodes are taken from holding ovens and left out in the atmosphere longer than the recommended time, then putting them back into a holding oven will not restore their low hydrogen properties. This is incorrect practice; they will not be restored to low hydrogen in holding ovens. Low hydrogen properties can only be restored when baked at higher temperatures in baking ovens, something which very few fabricators have. Excellent guidelines for electrode storage are given in CSA W59 鈥淲elded Steel Construction.

Electrodes that have cellulosic and rutile basic fluxes, typically E4211 and E4924 are not low hydrogen.

Regarding flux cored and metal cored electrodes that are classified with a CH number, they do not have to be controlled like the SMAW electrodes as the flux, which contains the hydrogen as moisture, is on the inside of the electrode and not the outside and, thus, not open to the atmosphere. These electrodes simply have to be kept dry and free from rust and foreign material.

Other sources of hydrogen come from the breakdown of water or organic material (such as grease, rust or paint) in the vicinity of the joint. These must also be controlled to limit HICC.

The susceptibility of the weld or HAZ to hydrogen embrittlement听

This is related to the chemical composition and final microstructure that is formed in the weld zone. This will be influenced by how fast the weld cools (cooling rate) and, depending on the ability of the steel to harden, will dictate the level of probability of HICC being formed.听

Steels with a certain hardenability will produce harder microstructures during cooling and this trend is generally accelerated as the strength of the steel increases. So, we can say the final听 microstructure depends on:

1. The heat inputto the weld (controlled by the weld procedure of amps, volts and travel speed)

2. The joint thickness ..........a 50mm butt weld will cool much faster than a 25 mm butt weld, all other things being the same.

So, if we know we have to slow down the cooling rate to reduce the hardness of the final microstructure, how do we do this?听 Well, we can do this by applying preheat. If preheat is called for in the weld procedure then the objective of this is to slow down the cooling rate and to soften the final microstructure to prevent HICC. A secondary objective of preheat is to help diffusion of any hydrogen away from the weld zone.听

听

In conclusion we can say that to reduce the probability of HICC we can:

• Select a low hydrogen welding process or low hydrogen welding consumable.

• If necessary, apply preheat to slow down the cooling (or quenching rate).听

• Control possible sources of hydrogen in the vicinity of the weld joint such as rust scale, moisture and grease on bare wires.

These items should be all covered in a comprehensive Welding Procedure Data Sheet (WPDS) or in a Welding procedure Specification (WPS) and should be accurately followed on the fabrication shop floor or in the field.

The reader is guided to Section 5 of CSA W59 and Appendix J of CSA W48 for further reading and guides on control of consumables, preheat levels and the importance of the CH 鈥渄iffusible hydrogen鈥� designation.