Mechanical properties of galvanized steels
Strength and Ductility
Extensive research has shown the galvanizing process has no effect on the strength and ductility of the structural steels commonly galvanized.
For steel to be in an embrittled condition after galvanizing is rare. The occurrence of embrittlement depends on a combination of factors. Under certain conditions, some steels can lose their ductile properties and become embrittled. Several types of embrittlement may occur but of these, only strain-age embrittlement is aggravated by galvanizing and similar processes. The following information is given as guidance in critical applications.
Susceptibility to strain-age embrittlement. Strain-age embrittlement is caused by cold working of certain steels, mainly low carbon, followed by ageing at temperatures less than 600°C, or by warm working steels below 600°C.
All structural steels may become embrittled to some extent. The extent of embrittlement depends on the amount of strain, time at ageing temperature, and steel composition, particularly nitrogen content. Elements that are known to tie up nitrogen in the form of nitrides are useful in limiting the effects of strain ageing. These elements include aluminium, vanadium, titanium, niobium, and boron. In Australia and New Zealand, these elements (excluding boron) are commonly included in structural steels.
Cold working such as punching of holes, shearing, and bending before galvanizing may lead to embrittlement of susceptible steels. Steels less than 3mm thick are unlikely to be significantly affected.
Hydrogen embrittlement. Hydrogen can be absorbed into steel during acid pickling but is expelled rapidly at galvanizing temperatures and is not a problem with components free from internal stresses.
Certain steels which have been cold worked and/or stressed can be affected by hydrogen embrittlement during pickling to the extent that cracking may occur before galvanizing.
Work by the GAA in 2017 showed that Bisalloy Grade 700 high strength steel to AS 3597 can be successfully hot dip galvanized. Like all steels to be hot dip galvanized, care should be taken with the edge treatment and thermal cut edges should be ground back to remove the thin oxide layer for best results. For these steels, the best results are achieved by abrasive blasting to a Sa 3 level of cleanliness prior to galvanizing and omitting the acid cleaning step to eliminate the risk of hydrogen embrittlement. The chemistry of this grade usually encourages a thick but adherent coating, providing exceptional durability compared to other grades.
High strength bolts to grade 8.8 and 10.9 are successfully hot dip galvanized all over the world. Grade 8.8 bolts are not affected by hydrogen embrittlement and no change in process is required. Best practice with 10.9 bolts usually includes minimal or no exposure to the acid pickling step, requiring the bolts to be cleaned by blasting or rumbling to Sa 3 level of cleanliness prior to galvanizing. (See the GAA’s Bolting Guide for more information)
The galvanizing process involves immersion in a bath of molten zinc at about 450°C. The heat treatment effect of galvanizing can accelerate the onset of strain-age embrittlement in susceptible steels which have been cold worked. No other aspect of the galvanizing process is significant.
Liquid metal embrittlement (LME) occurs when a combination of steel characteristics, fabrication detailing, and galvanizing processing variables create conditions for brittle cracking of a steel article during galvanizing. Such a combination of factors rarely occurs in practice. Control of the design (e.g., location of stress concentrations) and detailing of the component (e.g. steel quality, levels of residual stress, quality of welding, and position and finishing of drilled or punched holes and flame-cut surfaces), and the galvanizing conditions (e.g. pre-treatment conditions, dipping speed and zinc melt constitution) can eliminate the risk of this condition occurring. If design and fabrication is carried out to the requirements of AS 4100 or NZS 3401.1 and AS/NZS 5131, then LME will not occur as all members of the GAA carry out hot dip galvanizing practices that fully conform to the requirements of AS/NZS 4680.
Recommendations to minimise embrittlement Where possible, use a steel with low susceptibility to strain age embrittlement. Following the requirements of AS 4100, NZS 3401.1 and AS/NZS 5131 will eliminate all embrittlement. Where cold working is necessary the following limitations must be observed:
- Punching. The limitations specified in AS 4100 and AS/NZS 5131 on the full-size punching of holes in structural members must be observed. Statically loaded members and connections in fatigue must not have punched holes in steel thicker than 12mm. Material of any thickness may be punched at least 3mm undersize and then reamed or be drilled. In CC1 and CC2 categories of AS/NZS 5131, holes may be punched full size in material where fy is up to 360MPa and where the thickness does not also exceed (5600/fy) mm.
- Shearing. AS/NZS 5131 does not permit steel thicker than 16mm to be sheared unless it is stress relieved prior to galvanizing, although typically, steel thicker than 16mm is thermally cut. Sheared edges to be bent during fabrication should have stress raising features such as burrs and flame gouges removed to a depth of at least 1.5mm. Before bending, edges should be radiused over the full arc of the bend.
- Bending. Susceptible steels should be bent over a smooth mandrel with a minimum radius 3 times material thickness. Where possible hot work at red heat. Cold bending is unlikely to affect steels less than 3mm thick.
- Critical applications. It is better to avoid cold work such as punching, shearing, and bending of structural steels over 6mm thick when the item will be galvanized and subsequently subjected to critical tensile stress. If cold working cannot be avoided a practical embrittlement test in accordance with ASTM A143 should be carried out. Where consequences of failure are severe and cold work cannot be avoided, stress relieve at a minimum of 650°C before galvanizing. Ideally, in critical applications structural steel should be hot worked above 650°C in accordance with the steelmaker’s recommendations.
- Edge distances of holes. AS 4100 does not permit holes to be closer than 1.75 the fastener diameter for sheared or flame cut surfaces, 1.50 the fastener diameter for machine cut, sawn or planed edges of rolled plate, flat bar or sections and 1.25 the fastener diameter for the rolled edge of a rolled flat bar or section. The minimum hole distance can also be affected by Clause 18.104.22.168 (ply in bearing) of AS 4100.
Research and practical experience show that the fatigue strength of the steels most commonly galvanized is not significantly affected by galvanizing. The fatigue strength of certain steels, particularly silicon killed steels may be reduced, but any reduction is small when compared with the reductions which can occur from pitting corrosion attack on ungalvanized steels and with the effects of welds.
For practical purposes, where design life is based on the fatigue strength of welds, the effects of galvanizing can be ignored. Fatigue strength is reduced by the presence of notches and weld beads, regardless of the effects of processes involving a heating cycle such as galvanizing. Rapid cooling of hot work may induce microcracking, particularly in weld zones, producing a notch effect with consequent reductions in fatigue strength. Section 11 of AS 4100 provides for details of fatigue design of steel structures.
In critical applications, specifications for the galvanizing of welded steel fabrications should call for air cooling rather than water quenching after galvanizing to avoid the possibility of microcracking and reductions in fatigue strength.