Cheshirechappie
Established Member
Doing a bit of background research on tool steels, I came across reference to this phenomenon in a book called 'Heat Treatment, Selection and Application of Tool Steels' by William E Bryson. I've since found a similar warning in a pamphlet published by Jones and Shipman (manufacturers of precision grinding machines) in 1975, particularly with reference to the off-hand grinding of HSS lathe tools, so it's been known about for a while.
Micro-cracks can (they don't always) occur when hard materials are ground aggressively (or 'abusively' as one reference described it). Off-hand grinding is classed as 'aggressive' grinding. What happens is that the temperatures generated by the grinding operation (up to 1650 degrees centigrade according to Bryson - that's why the sparks come off white) cause localised expansion of the surface of the material. The steel can't expand sideways because cold metal restrains it, so it expands outward, and inward as far as the plasticity of the material allows. Then it cools, and the steel tries to shrink back to it's original position. Since some plastic deformation took place in the hot state, it can't fully shrink back now that it's cold, so high stresses are generated locally in the metal skin, some high enough to result in localised fracture. The localised heating also alters the metal hardness, and the fine metallurgical grain structure in the surface layers will be destroyed.
Micro-cracks cannot be detected by the naked eye, or by microscope. They can be found with magnetic particle or flourescent particle testing, and are between 0.002" and about 0.020" (0.05 to 0.5mm) deep. They are very difficult to remove once formed, since every time you grind over them, they heat up again and run deeper. (They could, I think, be removed by honing off the tool edge.)
The harder the material, the more likely the phenomenon is to occur. Bryson estimates that probabiity of occurence is about 30% if a straw colour is observed on the surface, and about 70% if a blue colour appears.
How to avoid it? Avoid the possibility of heat build-up by using a friable or semi-friable wheel, and a light touch in grinding. Wet grinding is better than dry grinding, and dunking a hot tool in water to cool it is best avoided - the sudden thermal shock can increase the chances of cracking - better to allow the tool to cool slowly if warmth is felt during grinding.
For edge tools, it probably isn't much of a problem if it happens well up the bevel, but would potentially cause early edge failure if it occurred at the cutting edge. The advice to grind most of the bevel but not right to the edge seems good advice.
It does occur to me that this MAY be a partial explaination for the edge brittleness reported with the harder steels like A2 in some circumstances. Extra care in grinding these steels might reduce problems of brittle edges, by avoiding the chances of micro-cracks forming near the edge.
Micro-cracks can (they don't always) occur when hard materials are ground aggressively (or 'abusively' as one reference described it). Off-hand grinding is classed as 'aggressive' grinding. What happens is that the temperatures generated by the grinding operation (up to 1650 degrees centigrade according to Bryson - that's why the sparks come off white) cause localised expansion of the surface of the material. The steel can't expand sideways because cold metal restrains it, so it expands outward, and inward as far as the plasticity of the material allows. Then it cools, and the steel tries to shrink back to it's original position. Since some plastic deformation took place in the hot state, it can't fully shrink back now that it's cold, so high stresses are generated locally in the metal skin, some high enough to result in localised fracture. The localised heating also alters the metal hardness, and the fine metallurgical grain structure in the surface layers will be destroyed.
Micro-cracks cannot be detected by the naked eye, or by microscope. They can be found with magnetic particle or flourescent particle testing, and are between 0.002" and about 0.020" (0.05 to 0.5mm) deep. They are very difficult to remove once formed, since every time you grind over them, they heat up again and run deeper. (They could, I think, be removed by honing off the tool edge.)
The harder the material, the more likely the phenomenon is to occur. Bryson estimates that probabiity of occurence is about 30% if a straw colour is observed on the surface, and about 70% if a blue colour appears.
How to avoid it? Avoid the possibility of heat build-up by using a friable or semi-friable wheel, and a light touch in grinding. Wet grinding is better than dry grinding, and dunking a hot tool in water to cool it is best avoided - the sudden thermal shock can increase the chances of cracking - better to allow the tool to cool slowly if warmth is felt during grinding.
For edge tools, it probably isn't much of a problem if it happens well up the bevel, but would potentially cause early edge failure if it occurred at the cutting edge. The advice to grind most of the bevel but not right to the edge seems good advice.
It does occur to me that this MAY be a partial explaination for the edge brittleness reported with the harder steels like A2 in some circumstances. Extra care in grinding these steels might reduce problems of brittle edges, by avoiding the chances of micro-cracks forming near the edge.
