question about grinding please

[Spinerazor]

Hey guys, i'm working on a blade and today when i was grinding it it all of a sudden changed color to like a dark grey and got incredibly smooth like glass, there are no perceptible scratches on it aside from one i put there to see if it would scratch and its smooth as glass, not sure why this happened, the belt seems to still have lots of grit left too, i was using a 120 grit belt and it was grinding like normal till this happened, very strange to me, did i burn the metal somehow? let me know what did this please!

 

[Stacy E. Apelt - Bladesmith]

Moved to Shop Talk.

You probably "Glazed" the surface. It got very hot and the surface colors changed ( called oxide colors). The heat melted some of the belt grit and made a super thin glassy surface. It can sometimes be pretty hard and take a little grinding to remove. This is usually caused by the belt running too fast and you using too much pressure while grinding. No damage on the blade if done before HT. If this happened after HT, the blade would need to be re-done or scrapped.

 

[Spinerazor]

that perfectly describes what seems to have happened, i cant slow down the grinder but i can surely ease up on my pressure, i'm working on a few blades today so i'll see if my technique changes the outcome. As for the blade that this happened to i have been sanding it this morning and wow its tough getting this layer off, it behaves exactly like smooth glass does! thanks for your help.

 

[Stacy E. Apelt - Bladesmith]

A good rule is to give the specs and techniques with any post or question. Knowing what type of steel you are working, and what type of sander you are using helps get a better answer.

If the steel you are grinding is one that work hardens, you may also have hardened the surface. O-1 and 1095 will both do this, as well as most stainless and other high alloy steels.

Switch to a fresh belt and sand the hardened surface away. Dip in water every few seconds and keep at it until the hard "glaze" is gone. If you have the ability, annealing the blade will help a lot.

 

[Spinerazor]

Hey Bladsmth, sorry its 1084 and i'm using a crappy 1x30 single speed 3200 rpm belt sander, after tweaking a bit it turned out ok i think and its ready for my attempt at heat treating now, wish me luck! and thanks again!