Published 1/15/1999

A Synergistic Approach To Machining Stainless Steel

One of the most interesting examples of this synergy comes out of its cutting tool and specialty steel operations. Sandvik has developed grades of stainless steel especially formulated for machinability along with cutting inserts especially designed for optimal performance with these workpiece materials.

Edited by: MMS Editorial Team

Sandvik Coromant is only one part of the Sandvik family of global companies and certainly the one best known to machine tool users because of its line of cemented carbide cutting tools and modular tooling systems. The other Sandvik companies include those involved in mining and construction, saws and hand tools, conveyor belts and related process systems, ceramics and other hard materials, and specialty steels. These businesses form a diverse but remarkably coherent group and top corporate management at Sandvik, which is based in Sweden, is always looking for productive synergies between its many companies.

On the one hand, Sandvik Steel formulated its SANMAC (Sandvik Machinability Concept) alloys to reduce the difficulties commonly encountered with stainless steels. In customer trials, the machinable 304 and 316 grade steels typically reduced machining time 20 percent with conventional tools. On the other hand, Sandvik Coromant developed a new breed of cutting inserts specifically for stainless (ISO M-class) alloys. With ordinary stainless steel, customers report productivity 10 to 20 percent better than conventional cutting inserts. Together these new alloys and inserts offer significant benefits for those machining stainless steels.

Machinable SANMAC 304 and 316 stainless grade, in regular and low-carbon compositions, have end-use properties identical to conventional 304 and 316 stainless steel, the company says. They often can be used as drop-in replacements. However, sulfide and oxide inclusions in the machinable stainless promote chipbreaking and have a lubricating effect at high cutting speeds. They also create a thinner work-hardened zone. The effects have been shown to be shorter machining time with higher feeds and speeds, improved tool life, and better surface finish.

One manufacturer machining 1.5-inch rings on a Mazak Quick Turn CNC machine compared SANMAC 316L stainless with a high sulfur 304 stainless. Type 316 is typically inferior to Type 304 stainless in machinability, but the SANMAC alloy was credited with a 20 percent reduction in machining time with no other processing changes. Spindle speeds with the new alloy reached 630 sfm for roughing and 1,120 sfm for the finishing pass, 40 percent higher than before with Type 304 stainless. The machinable stainless also improved formation of more discrete chips. Chips with the conventional stainless steel forced operators to clean the machine after each part adding 5 to 10 percent to the floor to floor time per part. After changing the workpiece material, cleaning was not necessary after every piece was completed.

The insert wear caused by conventional stainless steel also changed the dimensions of finished parts. Parts machined from SANMAC stainless had more consistent dimensions and a better surface finish than those machined from ordinary stainless steel.

Much as a more machinable stainless steel hikes productivity, cutting inserts optimized for stainless alloys shorten part cycles and cut machining costs. Sandvik M-Line tungsten carbide inserts have their composition, geometry, and special coatings tailored to stainless steel and were developed with SANMAC stainless steel in mind. However, they have demonstrated enhanced performance in other difficult stainless steel grades.

M-Line inserts come in rough-, medium-, and finishing geometries, all cobalt-enriched at the surface to strengthen their cutting edges. The chipbreaker design helps keep the workpiece free of stringy, segmented chips. The positive-rake geometry also promotes a soft cleaving-type cutting action that extends tool life and improves surface finish, Sandvik developers say.

New coatings give M-Line inserts a protective heat shield that minimizes built-up edge and puts heat into chips and coolant, not the insert. Titanium carbon-nitride and aluminum oxide are applied in a durable base coat, then the insert is topped with titanium nitride. The strong coating adhesion enables the machine operator to run faster or slower to increase productivity or decrease tool wear. The coating also promotes low-friction chip flow over the insert. Easy chip flow reduces cutting forces and tool wear, and helps improve part finish.

A double-lobed impeller rough turned in cast 316 stainless steel on an Okuma LB15 lathe took 7.27 minutes to machine with conventional inserts. Machining time with M-Line inserts optimized for stainless steel was 2.64 minutes. Cutting speed increased from 225 to 450 sfm and spindle speed from 107 to 214 rpm. Productivity increased 79 percent. Insert life increased four-fold, from two inserts per workpiece with conventional inserts to two work-pieces per insert.

Stainless steels are finding increasing use in material processing, power generation, transportation, and other applications. The challenges of cutting austenitic stainless can be overcome with both good machining technique and advanced material and insert technology. Machinable stainless and optimized cutting inserts can be used separately to increase productivity and cut costs., or together for a double boost in benefits.