This group of materials contains hardened and tempered steels with hardnesses >45 – 68 HRC. Common steels include carburizing steel (~60 HRc), ball bearing steel (~60 HRc) and tool steel (~68 HRc). Most of these steels are applied in mold & die industries. Hardened steel is the smallest group from a machining point of view and finishing is the most common machining operation. Specific cutting force: 2550–4870 N/mm² with hardness 60-68 Hrc. The operation usually produces fair chip control. Cutting forces and power requirements are quite high.
MATERIALS Hardened Materials
Hardened and tempered materials with hardnesses >45 – 68 HRC.Transmission shafts, gear box housings, stamping dies, etc.Machining success depends a lot on clamping stability, as cutting forces are very high.
Hardened steel applications – Mold & die
Hardened steel machinability
Due to the hardnesses > 45 – 68Hrc the machinability of these materials is considered difficult. The strength on these material with 45 Hrc can achieve 1500 N/mm2
Normally is applied carbide coated inserts on PVD (TialN) and substrate of submicron grain. The application must use low cutting speed, more feed and low DOC, this is the best way to get better performance and high feed inserts are the more suitable.
The PVD coated inserts combined with submicron grain can support more cutting pressure machining hardened steel and it’s desirable to apply shortest the tool overhang as possible, in order to avoid vibrations effect using very rigid tool fixturing (Weldon or precise collets) as well.
Hardened steel challenges
Let's have a look at the report of a test with high-feed machining a mold & die part
APKW 0602- HF LT3000
Company: Optimum mold
Machine: DMG Mori Seiki
Competitor cutter diameter 12mm Z= 2 - 2 cutting edges Lamina cutter diameter 12mm Z= 3 - 2 cutting edges
Material: P20 45Hrc
Key success factors
• Selection of the right insert: The reinforced cutting edge of the APKW insert, is suitable for the hardened steel application.
• Right carbide grade: LT 3000 with its submicron grain and AlTiN PVD coating has excellent performance machining Hardened steel.
• Dry operation using air blowing to remove the chips from the cutting zone.
• It was applied more DOC in order to improve the chip removal rate, and bit more Vc.
Result: Lamina insert improved the chip removal in 116%
Possibilities of improvement
If the first result was not satisfactory, what else could we try to improve the performance?
• Try lower Vc, compensating metal removal with more feed rate. In difficult to machine materials, such as Hardened steel, even a small change in speed can make huge differences in tool life. e.g. 10% lower Vc and 10% higher feed.
• Apply the grade LT3130.
• Use conventional milling direction, in some cases it minimizes chipping and vibrations.
Hardened steel is a difficult material to cut, but we can get excellent results if we select the right tool and set the right parameters. Remember it is important to select a reinforced geometry and cutting edge, try always to get a very rigidy set up , using the shortest overhang as possible and rigidy fixturing that brings excellent run-outs , axial and radial. Apply when is possible, the air blowing for better chips evacuation, mainly for cavity operations.