Broach Tool Materials
Almost all broaches are made of high-speed tool steels in monolithic construction. Brazed carbide or disposable inserts are sometimes used for cutting edges, most often on tools used for broaching cast irons.
Here is a list of tool steels and the materials that are commonly broached with these steels. (The list is only a sampling.)
M-2 steel:
Part hardness should be held under Rc 28. General use, including brass, aluminum, magnesium, and the following steels: 1018, 1020, 1063, 1112, 1340, 1345, B-1113, 4140, 4340, 5140, 8620, (RC26), 347 annealed stainless steel
M-3 steel:
Part hardness should be held under Rc 28. General use, aluminum castings, cast irons, A-286 and the following steels: 4140, 4337, 8617, 8620, 9840, 403 stainless, Greek Ascoloy, M-252, D-279, 4140, 4337, 4340, 8617, 8620, 9310, 9840, 403 stainless
PM-4 (Powdered Metal):
Part hardness should be held under Rc 30. An increasingly popular tool steel used on a wide variety of applications. Has a very high wear resistance. High Silicon Steels, Silicon Bronze, Aluminum Die Casting, Armature Grade Irons, 9250, 9260, All materials listed under M-2 & M-3 above.
T-15 (Powdered Metal):
One of the best and most expensive tool steels., Aluminum 2219, A-286 (Rc 32-36), Stellite, 17-22A(S)(Rc 29-34), N-155 (Rc 30-40), WASPOLOY, INCOLOY SOL (Rc 32-36), 4340 (Rc 30-40), 52100, 931- (Rc 26-30), 17-4 PH stainless steel, 416 stainless steel (Rc 35-40), 403 stainless steel (Rc 37-40), Custom 450, High Nickel, 4337 (Rc 29-34), 9310 (Rc 36-38), 9840 (Rc 32-36), Greek Ascoloy
Carbides
Most of the carbide cutters used to broach cast iron are used in flat surface broaching applications, although contoured cast-iron surfaces have been broached successfully. Surface broaching of pine tree slots has been tried with carbides on high-temperature alloy turbine wheels, but with little success. The carbide edges tend to chip on the first stroke.
Carbide-tipped broaches
Carbide tips are seldom used on conventional steel parts and forgings. One reason is that good performance is obtained from high-speed-steel tools; another is the low cutting speeds of most broaching operations (from 12 to 30 fpm) do not lend themselves to the advantages of carbide tooling. The success of carbide tooling on cast irons is due to carbide's resistance to abrasion on the tool flank below the cutting edge.
Another problem with carbide-tipped tools is that a broaching machine work fixture must be exceptionally rigid to prevent chipping of the cutting edge. Experimental work with extra-rigid tools and work piece fixtures, however, has shown that tool life and surface finish can be greatly improved with carbide tipped tools, even when used on alloy steel forgings.
Cast high-speed tool steels are almost never used in broaches. One property of the cast tool materials that prohibits their use in monolithic internal pull broaches is low tensile strength. Most cast alloys that can attain a hardness of Rockwell C 60 or higher do not have ultimate tensile strengths much in excess of 85,000 psi.
SURFACE TREATMENT
There are several practical ways of extending the life of a broach tool. One can be the use of surface treatment, such as nitriding, TICN, TIN, oxidation, or hard chrome plating, to increase the surface hardness and wear resistance of the broaching tool work piece. The return on the investment of coatings must be evaluated on a case by case basis.