Background From time to time, galvanisers get inquiries about galvanising special steels or unusual steel sections that are outside the normal range of steels processed on a day to day basis through the company's galvanising operations. While these steels can present problems for galvanising, it is sometimes possible to accommodate these problems in the design of the components or through modifications to the hot dip galvanising process. Galvanising Alloy Steels While high strength alloy steels are rarely used for structural applications, they are sometimes uses as performance critical components in assemblies or as individual manufactured items. There are three factors that affect the ability of steel to be galvanised. These are:
1. The chemical composition of the steel
2. The strength rating of the steel (the yield strength in MPa)
3. The steel's section thickness*
* This is a factor in that it determines the immersion time of the steel in the molten zinc.
When a request is received to galvanise an unusual type of steel, the chemical composition is first checked. Most special steels contain carbon (C), phosphorous (P), manganese (Mn), silicon (Si), sulfur (S), chrome (Cr), nickel (Ni) and may also contain copper (Cu), Vanadium (Va) and other elements that are used to give the steel particular performance characteristics.
What is in Alloy Steels?While there are hundreds of different types of special steels, there are generic chemistries that fit particular applications and these are useful in determining what is likely to happen when these steels are hot dip galvanised. The following list is a guide to the most prominent alloying elements likely to be in these special steels that will have an effect on their ability to be galvanised:
· Spring steels: These steels contain high levels of silicon which can be up to 2.0%.
· Tough steels: These steels contain high levels of manganese which may be over 1.0%
· Hard steels: These steels contain high levels of carbon which may be over 1.0%
· Free machining steels: These steels contain high levels of sulphur
· Electrical steels: These steels contain high levels of phosphorous.
· Stainless steels: These contain high levels of nickel and chrome.
The galvanising characteristics of these steels are as follows:
High Silicon SteelsHigh silicon steels will produce thick galvanised coatings which may be brittle because the steel reacts very rapidly with the zinc. The effects of high silicon content can be minimized by keeping immersion time in the zinc as short as possible. This becomes increasingly difficult as section thickness increases.
High Manganese SteelsHigh manganese steels will produce brownish coloured coatings that may be brittle and easily damaged in handling compared to galvanised coatings on conventional steels.
High Carbon SteelsHigh carbon steels can be successfully galvanised as long as their yield strength is within an acceptable range.
High Sulphur SteelsHigh sulphur steels are used for high speed machined components (threaded fasteners, sockets etc) and should not be galvanised. The high sulphur steel can be severely eroded in the galvanising process, rendering threaded items unserviceable.
High Phosphorous SteelsHigh phosphorous steel are rarely encountered in galvanising operations but are unsuitable for galvanising. They react rapidly with the zinc to form thick, dark coatings that are easily damaged and may delaminate from the surface.
Stainless steels can be galvanised but are susceptible to liquid metal embrittlement and can fracture under load after immersion in molten zinc. Stainless steels are only galvanised incidentally if they are attached to mild steel assemblies.
Steel Strength and GalvanisingHigh strength steels (around 1000 MPa yield strength and over) are susceptible to hydrogen embrittlement arising from the pickling process in galvanising. Pickling should be avoided for steels in this strength range.
Steel Size and GalvanisingWhile all conventional steel structural sections can be galvanised, from time to time, unusual sections arise that may present problems. Very thick sections over 100 mm in thickness may be difficult to galvanize acceptably in a conventional galvanising bath. The mass of these items per unit of volume is very high, and as the zinc in the galvanising bath is only about 35 degrees above its freezing point, the zinc freezes around the item when it is immersed, and may form a layer of frozen zinc 50 mm or more in thickness.
This zinc has to be re-melted and then the item itself heated up by the zinc bath to bath temperature for the galvanised coating to form. This sequence of events may interfere with the performance of the flux on the surface of the item and cause uncoated areas on the surface.
These defects can be minimized by pre-heating the item or operating at higher galvanising bath temperatures, which requires special galvanising bath design.
ConclusionIt is possible to successfully galvanise difficult steels, particularly spring steels which are the type most likely to turn up for galvanising. As long as the section thickness is not sufficient to be the determining factor in immersion time, and the springs can be abrasive blasted rather than pickled, an acceptable galvanised coating can be produced on certain types of coil and leaf springs that will not affect their performance.