Diffusion Bonding of 400 Series Stainless Steel Blanks
Furnace Run and Preparation
This job was to diffusion bond a pair of 400 series stainless steel blocks and then heat treat them.
Prior to stacking in the furnace, we applied load film to each of the faying surfaces and applied about 7,000 pounds of force (300psi) to activate the load film. We then viewed the contact pattern between the faying surfaces and determined the parts were making adequate contact for us to proceed with the bond run.
There is a double layer of oxide barrier between the parts and the graphite platens. Because 400 series stainless forms a eutectic with graphite, which is pure carbon, without this barrier we would melt the stainless steel accidentally, ruining the parts and the graphite platens. For this run, the layers consisted of a stainless steel separator sheet sprayed with Boron Nitride and then clean fired to promote oxidation, and a glass cloth/fabric material.
In the center of the stack is a Molybdenum plate, that is flat within .001”. We use Molybdenum for this purpose because it maintains its strength at high temperature and has a very high melting point of about 2600 degrees Celsius.
Cryo Freeze and Hardness Testing
After the bond run was complete we unloaded the parts from the furnace and were allowed to cool enough for us to handle. We then moved them into the cryo freezer. The cryo freezer was pre-chilled to -78 degrees Celsius.
The purpose of Cryogenically freezing the parts was to convert any retained austenite within the crystal structure of the steel into Martensite.
We did hardness testing after this process and got about 55 HRC.
Tempering and Final Hardness Testing
Post Cryo Freeze the parts were loaded in an air oven and tempered.
After tempering the parts were hardness tested again. We ended up with a final hardness around 52 HRC.