Advanced Thermal Treatment of Carbide Material
While the science of cryogenic processing is still evolving, it is science and not science fiction. Unfortunately, some who promote cryogenic processing don't have a fundamental grasp of metallurgy and the science of working with steel. This has caused some confusion in the marketplace. There has, however, been substantial work describing the benefits of cryogenic processing, and some of that work is summarized in the ASM Handbook Volume 4 Heat Treating.
Several changes in the microstructure of the carbide seem to be the causative factor in improvement of carbide durability as a result of advanced thermal treatment. These changes are the principle reasons for the dramatic improvement in wear resistance:
- First, particles of existing carbide are refined into their most stable form. This “spheroidization” also aligns the carbide particulate structure into a durable, stress free configuration. This reduces the risk of stress-induced fractures.
- Second, fine eta(n) carbide particles (precipitates) are formed during the long exposure to critical temperatures (chromium carbides, tungsten carbide, etc., depending upon the alloying elements). These are in addition to the larger carbide particles present before thermal treatment. These fine particles or "fillers", along with the larger particles, form a denser, more coherent and much tougher matrix in the material.
- Third, stresses in the material (residual from the manufacturing process) are relieved. These micro-stresses are the leading cause of early carbide fracture. The stresses of manufacture, and bonding to the substrate are relieved throughout the material, permanently.
Some research has been done directly relating to carbide changes at cryogenic temperatures.
"Tungsten carbide with cobalt binders will extend in life by as much as 1,000 % when treated properly with [computer controlled] cryogenics. Tungsten carbide stress relieves and also refines its grain structure even though it is micro-grained already."
(R.F. Baron, Professor Emeritus Louisiana Polytech, conveying information from a university study done in Arizona, by phone 3/27/2003)
Also, the Arizona State study conducted by Laurel Hunt, used deep treated C-2 de-burring tools on INCONEL alloy 718, achieving a 400% improvement based on weight, after five cuts of .003 in. (.007 cm) on this alloy. This deep cryogenic treatment of an 8% cobalt end mill has made dramatic improvements in two important ways. The number of milling cuts was increased from three before deep cryogenic processing, to 78 cuts after processing (26 times the wear life). Re-sharpening the end mills after deep cryogenic treatment required only 1/3 the amount of stock removal to restore the tool geometry.
While physics still seeks to document the fundamental effects of cryogenic treatment on carbides, some of the most startling proof is already realized in direct applications of carbide-tipped tools. Areas of benefits include:
"Inserts, routers, roughers, endmills and other tool configurations are all very cost effective … Carbide tipped concrete saws and asphalt profilers can see a gain of 100% in life as well as continuous miner bits, Tungsten carbide impregnated crusher cones and long wall bits used in mining."
"… bandsaw blades […] used to cut 4133 steel. Before tempering, each blade cut about 625 pieces. After treatment, blades were averaging 1,500 to 1,600 pieces. "We were using 36 blades a month, and we cut it down to 14 to 16 blades a month, maintaining the same productivity,"[…]
"Drill inserts improved to 210 pieces off each insert, from 131, after cryogenic tempering. The biggest improvement was seen with cutting inserts. Prior to tempering, cutting inserts were averaging 720 pieces per cutter. After tempering, the life increased fivefold, to 3,500 to 4,000 pieces per cutter, […] now tempering thread mills, used to produce threads on cylinders, although it is too early to quantify results."
Rockwell, a major aircraft manufacturer, using C-2 carbide inserts to mill epoxy graphite, doubles their output after deep cryogenic treatment of the inserts. In a second test, a 400% improvement was achieved upon milling 4340 stainless steel with a cryogenic-treated tool.
While cryogenics shows some promise in improving the metallurgical properties of carbide material, it is only a small part of the equation. Drastic improvements are only realized when complete thermal processing is combined with accurate computer control. Thermal processing of carbides is already showing dramatic, real-world benefits.
In current testing using local drilling companies, a 7-inch drag bit with the expected lifespan of 800 feet of bore depth drilled through just over 4,000 feet. Four hundred of those feet were through chirt, a material that should have destroyed the bit. Also, the bit did not have to be sharpened as often as normally experienced by this driller. Conservatively, that is a 400% usable lifespan improvement in the bit, effected through advanced thermal processing. Not to mention that 10% of the lifespan improvement was in an environment that would have been fatal to the tool otherwise.
Thermal Processing of Carbides Bonded to Metallic Substrates
Due to the high temperatures involved in bonding carbide materials to metallic substrates, it is recommended that thermal processing only occur after such bonding.
Bonding temperatures normally associated with bonding carbide cutting surfaces to steel or other metallic substrates typically exceed 1000ºF. This is well above the critical temperature of most metals and alloys. This bonding process creates stresses at the bonding event layer in both the materials. These stresses essentially are created at the time of bonding, so must be relieved after the bonding process in order to provide the maximum strength and durability potential.
Thermal processing of the individual parts before the bonding process imparts the benefits of the processing to those parts, but not to the material at the bonding layer. This material must also be treated thermally, or it becomes a “weakest link” point of failure for the entire tool.
Mounting, real-world evidence shows vast improvements in the performance of carbide materials derived from computer controlled advanced thermal processing. Metal Science Services stands by to assist you with all your heat treatment needs.