Specifying bead and shot blasted stainless steel finishes and their applications

Introduction

Bead and shot-blasted finishes produced by the impact of a hard, inert medium onto the steel surface result in non-directional, uniform matt surfaces with low reflectivity.

These finishes have both aesthetic appeal and can enhance the surface properties of stainless steels. They produce similar finishes to those obtained by acid etching. Blasted material can be chemically coloured, combined with rolled patterns, masked and polished or acid etched to create standard or bespoke designs.

The texture of the blasted surfaces vary with the blast media, which include glass, ceramic or lead bead, silicon carbide, aluminium oxide, stainless steel shot or ground quartz. These finishing methods can be used in the specialist finishing shop or under site conditions, i.e. either before or after fabrication.

For architectural applications steel shot or glass bead peened finishes are a good choice for getting a good combination of corrosion resistance from a non-directional surface ‘sheen’ finish.

Comparison of blasting and peening

In blasting the impacting medium cuts the steel surface, removing small amounts of metal from the surface. The resulting surface finish and hence surface corrosion resistance of the treated stainless steel is partly dependant on the blasting medium. Very hard media, such as aluminium oxide can leave a jagged or torn faces to metal peaks on the surface, whereas the softer media, such as silicon carbide give a smoother finish. The aluminium oxide finish can result in reduced corrosion resistance and cleansability compared to silicon carbide of a similar grit size.

In contrast, peening a surface does not remove metal. The impact of the rounded bead blasting medium results in small craters in the surface, giving a dimpled appearance, at a suitable magnification, as metal is pushed aside. The edges of these small craters are not however sharp and so the corrosion resistance and cleansability of peened surfaces compares well with blasted surfaces, for a given steel type and ‘roughness’ of the surface.

It is important that only clean round beads are used in the peening process.

A build up of broken beads in the peening medium can result in an undesirable uneven surface finish being produced and the excessively rough surfaces produced could be prone to staining or micro-pitting in service. The surface of the steel being treated should be cleaned before and after blasting.

Properties of bead blasted finishes

Bead blasting results in a non-directional textured surface with a soft satin reflection and low reflectivity.
This process works well when applied over satin or mirror polished finishes.

A range of coarsenesses can be created, depending upon the media used and the thickness of the material treated. Unlike acid etching, no material is removed from the surface during the blasting process, but the character of the surfaces produced by these two processes are similar.

During peening the surface of austenitic stainless steel is work hardened and surface compressive stresses are induced. There are specific advantages from these surface property changes:
Improved fatigue resistance
Increased surface hardness. The finish has enhanced scratch resistance.
Improved stress corrosion cracking resistance of austenitic stainless steels, (for any given steel type and service environment.)

The surface work hardening can however result in distortion. For this reason blast finishes should not be applied to stainless steel thinner than 0.4 mm. To avoid product distortion on thin sections, blasting on both sides should be considered.

Specifying bead blasted finishes for stainless steel

Bead blasted finishes for stainless steels are not covered in BS EN 10088-2 as a special finish type, (Table 6). Finishes can be specified by considering the blast media, the blast intensity and the coverage of the surface, but should be agreed with specialist surface contractors.

Blast Media

The Architects Guide to Stainless Steel shows the finish characteristics by blast media as follows:

Medium	Surface finish
Sand	Dark, coarse
Glass bead	Light, smooth
Silicon carbide	Very dark, coarse
Stainless steel shot	Honed
Ground quartz	Shiny, coarse

Blast Intensity
Control of the energy of the stream of the blast media is important for ensuring consistency of the finish. This is monitored through the deflection (arc height) of steel test strips that are only blasted on one side.

Coverage Control

Complete coverage of the surface with the peen craters has to be achieved for optimum performance of the finally treated surface. The original surface must be completely obliterated by the peen dimples. The coverage can be monitored by the finishing contractor using fluorescent tracer dye techniques.

The time to achieve 100% coverage can be exceeded and is then expressed as a percentage of that time e.g. 150% or 200% coverage.

At the ‘Member Products and Services’ page type ‘bead’ in the ‘Find What’ box with the ‘Service Supplier’ button selected. Click on the ‘Search’ button with ‘Bead blasting-Surface Finishing’ highlighted to view a contacts list.

Risk of contamination staining or rusting in service

For optimum corrosion performance, fine media should be used which do not embed into the surface. As with other mechanical finishes, media must be kept free of iron contamination.
Steel shot should not be used as a blast medium on stainless steels.
Sand blast media can contain iron / steel contamination and by preference should not be used.
Glass beads are the preferred blast medium.

Applications for bead blasted stainless steel

Applications for blasted stainless steel finishes include structural support members, (e.g. external walkway support arms). Cast glazing fixing and connections. Architectural external and internal cladding, (facades, columns etc.), sculptures and street furniture etc.