Mechanical Properties of Galvanized Steels

MECHANICAL PROPERTIES OF GALVANIZED STEELS

The galvanizing process has no effect on the mechanical properties of the structural steels commonly galvanized.

STRENGTH AND DUCTILITY

The mechanical properties of 19 structural steels from major industrial areas of the world were investigated before and after galvanizing in a major 4-year research project by the BNF Technology Centre, UK, under the sponsorship of International Lead Zinc Research Organization. Included were steels to Australian Standard 1511 grade A specification, and British Standard 4360 series steels.

The published BNF report ‘Galvanizing of structural steels and their weldments’ ILZRO, 1975, concludes that ‘… the galvanizing process has no effect on the tensile, bend or impact properties of any of the structural steels investigated when these are galvanized in the “as manufactured” condition. Nor do even the highest strength versions exhibit hydrogen embrittlement following a typical pretreatment in inhibited HCl or H2SO4. ‘Changes in mechanical properties attributable to the galvanizing process were detected only when the steel had been cold worked prior to galvanizing, but then only certain properties were affected. Thus the tensile strength, proof strength and tensile elongation of cold rolled steel were unaffected, except that the tensile elongation of 40% cold rolled steel tended to be increased by galvanizing. 1-t bends in many of the steels were embrittled by galvanizing, but galvanized 2-t and 3-t bends in all steels could be completely straightened without cracking.’

In 2010 this work was reviewed with structural steels in common use in the 21st century and reconfirmed the results from 1975.

EMBRITTLEMENT

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.

Cold working such as punching of holes, shearing and bending before galvanizing may lead to embrittlement of susceptible steels. Steels in thickness less than 3mm are unlikely to be significantly affected.

The galvanizing process involves immersion in a bath of molten zinc at about 450°C. Heat-treated or cold-worked steels can be tempered by the heat in the hot dip galvanized bath and lose some of any increased strength obtained by heat treatment or cold working. The heat from 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.

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. If steels are harder than approximately 34 HRC, 340 HV or 325 HB (approximately equal to a tensile strength of 1100 MPa), care is necessary to minimize hydrogen absorption during pre-treatment.

Recommendations to minimise embrittlement

Where possible, use a steel with low susceptibility to strain age embrittlement. Where cold working is necessary the following limitations must be observed:

Punching. The limitations specified in AS 4100 and AS/NZS 4680 on the full-size punching of holes in structural members must be observed. Material of any thickness may be punched at least 3 mm undersize and then reamed, or be drilled. Good shop practice in relation to ratios of punched hole diameter to plate thickness, and punch/die diametral clearance to plate thickness should be observed.For static loading, holes may be punched full size in material up to 5600/Fy mm thick where Fy is material yield stress up to 360MPa.
Shearing. Edges of steel sections greater than 16 mm thick subject to tensile loads should be machined or machine flame cut. Edges of sections up to 16 mm thick may be cut by shearing.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 read heat. Cold bending is unlikely to affect steels less than 3 mm thick.
Critical applications. It is better to avoid cold work such as punching, shearing and bending of structural steels over 6 mm 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 650ºC 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. In accordance with Australian Standard 4100 ‘Steel structures’, minimum edge distances from the centre of any bolt to the edge of a plate or the flange of a rolled section should be used.

FATIGUE STRENGTH

Research and practical experience shows 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.

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.

PROCESS