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Metal Casting Technologies : September 2006
44 www.metals.rala.com.au TECHNICAL FEATURE the common HPDC die steels H11 and H13, is one such example and is used for hot extrusion and other hot forming dies. Steel Grade P20 is a low alloy steel and is classified as a Low C Mould Steel, its main application is for the forming of polymeric materials as in plastic injection moulding although it is used for dies for HPDC of Zinc Alloys where the temperatures and service demands on the die are not so severe as for Al Alloys. It can also be used for prototype dies for Al casting. Other low alloy steels are also used for Zn diecasting dies based on compositions around 0.45-0.5%C, 1%Cr, 1.5%Ni, 0.4%Mo. Although Grades H10, 12, 19, 21, 23 and other variations of Cr, Mo, V, and W combinations have been used, Grades H11 and H13, which use Mo instead of W for secondary hardening, have become the workhorse steels for HPDC dies for Al Alloy casting. The higher Silicon levels in H10-13 give improved resistance to oxidation but some grades now have lower %Si for improved toughness. In all steels %S and %P levels are kept below 0.02, but some steel grades are resulphurised for better machinability. H11 (JIS SKD6) and H13 (JIS SKD61) have high hardenability and can be through hardened with minimal dimensional changes in relatively large sections, up to 150 mm., by cooling in air. During tempering Mo produces a finer dispersion of alloy carbides than W resulting in more uniform secondary hardening. Although W containing grades are more resistant to softening and erosion in service they tend to be more prone to cracking at hardness levels of 45-55HRC. Although quenched hardness levels of 50-55 HRC can be achieved the usual working hardness for dies after tempering is 44-48 HRC. IMPORTANCE OF STEEL PRODUCTION ROUTE Tool Steel buyers in countries like Thailand are sometimes confused by different "grades" or "qualities" of steel produced to the same alloy specification. They often complain "Surely H13 is H13? -- Why do some "grades" cost more? -- Are fancy trade-names just for advertising?" To answer such questions explanations need to be given about: ■ Improvements in steel making that result in very clean homogeneous steel with very low impurity levels. ■ The use of Powder Metallurgy as an alternative production route. ■ The extent of R&D performed by steel producers in fine tuning alloy compositions within existing specification ranges or in modifying these specifications to produce their own patented alloy grades. This work also includes optimization of hot working and subsequent heat treatment variables to achieve improved service performance. For improved toughness and resistance to both mechanical and thermal fatigue all steels need to be as "clean" as possible. "Clean" means freedom from non-metallic inclusions, minimum levels of damaging impurities such as Sulphur and Phosphorus and minimum dissolved Hydrogen and Oxygen. Inclusions and elements like S and P reduce both fatigue resistance and fracture toughness (7). Fortunately the development of secondary processing methods such as Vacuum Arc Refining (VAR) and Electroslag Refining (ESR) has enabled alloy steels with very low inclusion and impurity levels to be produced, especially if Vacuum Induction Melted (VIM) Steel Grade C Mn Si Cr Ni Mo W V Co P20 0.28- 0.60- 0.20- 1.40- _ 0.30- _ _ _ 0.40 1.00 0.80 2.00 0.55 H10 0.35- 0.25- 0.80- 3.00- 0.30 2.00- _ 0.25- _ 0.45 0.70 1.20 3.75 max 3.00 0.75 H11 0.33- 0.20- 0.80- 4.75- 0.30 1.10- _ 0.30- _ 0.43 0.50 1.20 5.50 max 1.60 0.60 H12 0.30- 0.20- 0.80- 4.75- 0.30 1.25- 1.00- 0.5 _ 0.40 0.50 1.20 5.50 max 1.75 1.70 max H13 0.32- 0.20- 0.80- 4.75- 0.30 1.10- _ 0.80- _ 0.45 0.50 1.20 5.50 max 1.75 1.20 H19 0.32- 0.20- 0.20- 4.00- 0.30 0.30- 3.75- 1.75- 4 0.45 0.50 0.50 4.75 max 0.55 4.50 2.20 4.40 H21 0.26- 0.15- 0.15- 3.00- 0.30 _ 8.50- 0.30- _ 0.36 0.40 0.50 3.75 max 10.00 0.60 H23 0.25- 0.15- 0.15- 11.00- 0.30 _ 11.00- 0.75- _ 0.35 0.40 0.6 12.75 max 12.75 1.25 H42 0.55- 0.15- _ 3.75- 0.30 4.50- 5.50- 1.75- _ 0.70 0.40 4.50 max 5.50 6.75 2.20 Table 2. Composition ranges in wt.% of alloying element for some grades of Hot Work Tool Steel. Data is from ASM Handbook (5, 6). Grades H11 and H13 are highlighted as the normal choice for diecasting dies. Grade P20 is a Low C Mould Steel grade that can be used for Zinc diecasting dies.