T4 is a cobalt-bearing tungsten high-speed steel, essentially T1's alloy system with roughly 4.25-5.75% cobalt added. Like T1, it's a true HSS grade made into the cutting tools themselves rather than into dies — drills, milling cutters, lathe tools, and other tooling that needs to hold a hard, sharp edge under sustained cutting heat. The cobalt addition is what separates T4 from T1 in practical terms: cobalt doesn't form carbides, but it raises the matrix's resistance to softening at elevated temperature, giving T4 meaningfully better red hardness and hot wear resistance than plain T1 at the cost of somewhat lower toughness and higher cost.

T4 hardens to a similar 63-66 HRC range as T1 but sustains cutting-edge hardness at higher sustained temperatures, making it the better choice for heavy-duty or continuous-cut applications generating more heat at the tool tip — roughing operations, abrasive materials, and production runs where T1 would soften and dull faster. This page covers T4 in its pre-hardened, machinable form: round bar and tool blanks turned, milled, drilled, and profiled into rough tool shapes before hardening and grinding finish the cutting edges.

T4 machines harder than T1 for the same reason T4 performs better in service: the cobalt addition raises resistance to thermal softening, which works against the tool at the cutting edge just as it works for the tool in the finished product. In the annealed condition (typically 235-262 HB, somewhat harder than T1's typical annealed range), T4's carbide population is similar to T1's — dominated by tungsten carbide with a smaller, very hard vanadium carbide fraction — but the cobalt-strengthened matrix resists the cutting edge more persistently, so cutting temperatures climb faster at a given speed.

Cutting forces and flank wear rates run above T1's, and chip formation is similarly short and segmented. As with all HSS grades, T4 is machined in the annealed state before hardening, with grinding handling the hardened cutting edges. Coated carbide grades with strong hot hardness, conservative feeds, and rigid setups are essential; pushing speeds to T1-level parameters will shorten insert life noticeably on this grade.