With mild steels that are produced by conventional methods, hot dip galvanising has no significant effect on their properties or performance other than enhancing the steel's durability and providing a low level of stress relief from fabrication stresses through the item’s being heated to the galvanising temperature.
Sources of Embrittlement
With some types of steels and with some fabrication techniques which involve severe cold working of the steel prior to galvanising, embrittlement problems can arise that can result in the performance of the item in service being affected.
Types of Embrittlement
There are three significant types of steel embrittlement that can be associated with the hot dip galvanising process. These are:
• Liquid metal embrittlement
• Hydrogen embrittlement
• Strain age embrittlement
Liquid metal Embrittlement
Liquid metal embrittlement is caused by the attack of the molten metal (zinc in the case of galvanising) on susceptible steels.
The most common liquid metal embrittlement problems associated with hot dip galvanising are with stainless steel. Attaching stainless steel fittings to mild steel items prior to galvanising should be avoided for this reason as the molten zinc may affect the mechanical properties of the stainless steel.
When atomic hydrogen diffuses into the structure of susceptible metal such as high strength steel, some mechanical properties can be seriously impaired. Sustained tensile stress can thus lead to failure. Dynamic and static laboratory testing can detect losses of tensile or torsional ductility.
Hydrogen embrittlement is caused by the presence of hydrogen atoms within the crystal lattice structure of a metal or alloy. In the galvanising process, hydrogen may be absorbed in the steel during the pickling process through contact with the hydrogen ions present in the hydrochloric acid.
Steels with a tensile strength in the order of 1000 MPa or higher or with an equivalent surface hardness of 30 Rockwell C or higher are considered to be most susceptible to hydrogen embrittlement.
The hot dip galvanising processes throughout Australia use hydrochloric acid at ambient temperature almost exclusively for pickling prior to galvanising. Acid concentration is typically 10-15% HCl.
The majority of steel hot dip galvanised is generally in the range of 200-450 MPa so is not subject to hydrogen embrittlement problems. Higher strength steels such as the quenched and tempered Bisalloy steels are appearing in the structural area and special consideration must be given to these types of steels if they are required to be hot dip galvanised.
Avoiding Hydrogen Embrittlement
The requirement to galvanise high strength steels is a very small one in comparison to the volume of lower strength product that is routinely processed through galvanising plants. High strength steels can be galvanised satisfactorily provided the necessary precautions are taken in the galvanising process.
The recommended method of processing high strength steels for galvanising is to eliminate the acid pickling process and use mechanical cleaning methods for preparation of the surface prior to hot dip galvanising.
Abrasive blast cleaning to Class 2 1/2 immediately prior to galvanising will ensure that the steel is adequately cleaned and that a satisfactory hot dip galvanised coating will be produced.
Australian Standard AS 1214-1973 Appendix C states the following with respect to hydrogen embrittlement of high strength bolts, which are the most commonly encountered high strength steel requiring to be galvanised.
Where additional sqfeguard is sought (eg. For bolts of Grade 10.9 or higher cleaned by acid pickling), fasteners should be baked at a temperature of 200°C + 10°C for a time found on the basis of experience to be adequate (for guidance, a time of 30 minutes before galvanising, or 4 hours immediately after galvanising, might prove satisfactory).
Strain ageing is associated with strain that results from plastic deformation which is more commonly known as cold working. Steel is an alloy of iron and carbon and contains other alloying elements which provide it with specific performance characteristics.
Severe cold working of steel causes the migration of carbon atoms in the iron crystals and the segregation of these atoms at dislocations in the steel causes a reduction in ductility of the steel.
The ageing process is a function of temperature and time and occurs very slowly at ambient temperature but very rapidly at the 450-460°C temperatures of the galvanising process. Severe cold working of steel can be caused by hole punching in thicker sections, tight radius bending or rebending.
It should be noted that it is not the hot dip galvanising that is the cause of accelerating the strain ageing of the steel, but the heat of the process, so strain age embrittlement can be induced in any severely cold worked steel by heating and the tendency to embrittlement by strain ageing will always be present and its manifestation will simply be a matter of time.
Avoiding Strain Age Embrittlement
To avoid the risk of strain age embrittlement, the following design criteria should be followed:
• Use bend radii at least 3 times the section thickness
• Hot bend if bend radii is under 3 times section thickness is required
• Anneal at 650-815°C prior to galvanising
• Ream punched holes to remove severely cold worked material from surface prior to galvanising.