TWI Industrial Member Report Summary 164/1981
By I A Bucklow, R J K Nicholson and D S Rhead
The rate at which surfaces of three irons and three steels lose their contaminants when heated to 500°C in high vacuum or when subjected to ion-bombardment was studied by X-ray photoelectron spectroscopy. Simple heating was not very effective in removing contaminants; ion-bombardment at room temperature was more effective than heating, but the cleanest surfaces were obtained by ion-bombardment at 500°C. It appeared that an increasing carbon content in the metal was associated with more efficient cleaning.
The rate of contamination of a cleaned steel surface was followed when exposed to carbon monoxide, water vapour, or ethylene at room temperature. It is probable that the steel surface was saturated with contaminant within 200sec, but there was also a significant degree of contamination from the vacuum atmosphere itself.
Ground steel surfaces heated in vacuum underwent smoothing by surface diffusion, but even at 900°C the effect was considered to be too localised to reduce significantly the interface void volume of subsequent bonds.
Bonding between lightly abraded and cleaned surfaces of a plain 0.4%C steel gave good joint tensile properties, but bondability was considerably impeded by prior exposure to carbon monoxide or ethylene (water vapour was not investigated); carbon monoxide was the more deleterious. Bonding between polished and cleaned surfaces of a 0.4%C heat-treatable steel at 700°C (i.e. within the tempering range) gave bond-zone strengths exceeding parent-metal strengths, but lightly-abraded surfaces caused the bond to fracture at the onset of necking. Bonding between mildly-abraded surfaces at 600°C (i.e. effectively without further tempering) gave joints which failed across the bond-line at the onset of necking.
The results indicate that the use of ion-bombardment for surface cleaning is entirely adequate and that, as a consequence, the factor controlling the tensile properties of joints produced below the A1 temperature is the presence of interface voids. The influence of bonding-face surface finish on tensile properties has not been resolved.
It is concluded that considerable success has been achieved in the bonding of steel below the A1 temperature. The report highlights the necessity to reduce even further the bonding temperature, especially for heat-treatable steel, and concludes that, as a result of the programme to date, the possibility of doing so is good.