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Ramax LH: Development of a New Mold Base Stainless Steel
By: Lena Rahlen, Plastics Application Manager And
Patricia Miller, Senior Technical Manager
For years, the focus of mold base development has been towards providing a steel with higher mechanical properties for the user of the mold bases. However, when these higher strength alloys are used, machining has not been optimized for the toolmaker, nor have his lead times or productivity. Several steel types have come into the market that lower the hardness of the steel, and hence improve machinability, but it is at the expense of other characteristics in the steel. A new mold base stainless steel has been developed that has an optimized alloy content which leads to greatly improved machinability as well as other important properties. Uniformity of hardness, and stability during machining have been optimized, while enhancing the corrosion resistance of the steel. In order to judge the value of a new steel, it must be compared with the most commonly used materials on the market. In the case of corrosion resistant mold base steels, the materials to compare with are: 420 Free Machining Stainless Steel, DIN Numbers 1.2085 and 1.2099. This article compares the new mold base stainless steel, called Ramax LH, with the properties of these steels. In today’s demanding times it is increasingly important to reduce time to market while improving productivity in the development of mold tooling. Due to the intensive machining practices that must be performed on mold bases, if machining time and costs can be reduced, then significant savings can be realized. There is a clear trend towards lower carbon content in prehardened material; this is valid both for corrosion resistant and non-corrosion resistant steels. The corrosion resistant mold base materials are no exception.
This grade has a leaner alloy composition than a typical 420 free machining stainless, which will maintain a good corrosion resistance but still give the desired machinability. Additions of Nickel, Molybdenum and nitritogen will also enhance the corrosion resistance. Also, tight control over process factors like ingot sizes and reduction ratio may cause differences in corrosion resistance, as the degree of chemical segregation may differ in the material and interfere with the corrosion resistance. Sulfur is present to aid in the machinability of the steel. Even the toughness level can be of importance for the machinability. A tougher steel is normally more difficult to machine. In Ramax LH the composition has been fine tuned in order to give the needed and wanted properties.
Hardness Hardness level does have an affect on strength and indentation resistance, but also on the machinability. So, to optimize the machinability while maintaining a good mechanical strength, a hardness range was established to accomodate the needs of the toolmaker and the end user. For example, Uddeholm currently supplies higher hardness grades to the marketplace in the range of 320-350 HB (~1080-1200 MPa). Now, Ramax LH is currently delivered in the range of 270-310 HB (~900-1050 MPa). The grade can be modified to provide higher hardness levels for key applications requiring it, while still having a machining advantage over current alloy types. Hardness range is important, but it is also as important for the mold base manufacturer and user that the hardness level is uniform throughout the entire dimension of the plate. The machinability can be reduced if areas with higher hardness are encountered during the machining operation, or the material in lower hardness ranges can be too gummy for machining. Uneven hardness may also result in less stable material. Below is an example of hardness profiles measured in 180 mm thick (7.09 inches) Ramax LH. Figure 1: Hardness profile in Ramax LH, thickness 180 mm
The hardness profile is very even, a difference of ~2 HRC in the 180 mm thick material. This uniformity will minimize gumminess in the material as it is drilled and tapped. It will also minimize hard spots in the steel from hardness variation.
Machining properties Machinability is the single most important operation determining the mold base making cost. It is however an extremely difficult property to test and to rank in a trustworthy way. Different cutting tools and parameters will give different ranking of tool steel so cutting parameters must be well defined for the tests. Even the toughness level of the steel can be of importance for the machinability. A tougher steel is normally more difficult to machine. In Ramax LH the composition has been fine tuned in order to give the needed and wanted properties. In house tests have been conducted in milling and drilling: cavity milling with coated carbide inserts and drilling with uncoated high speed steel drills. Some results can be seen in Figures 1 and 2 below.
Figure 1: Milling with coated carbide insert: The aim of the test is to find the highest possible cutting speed that will achieve 5000 cm3 of removed metal before the milling cutter is worn out(VBmax=0.5 mm). Whether at the higher or lower end of its hardness range, Ramax LH has developed very goood cutting speeds.
Figure 2: Drilling with uncoated HSS drills. The aim of the test is to find the highest possible cutting speed that will achieve 1m of total drill length before the drill is worn out. Drill used: Wedevåg Double-X, Ø 0.5 mm. The combination of chemistry, production processing, and hardness have improved the drilling capabilities of Ramax LH.
Gundrilling tests were also performed. Gun drilling is a very difficult and sensitive operation and has a big influence on the machining costs. Parameters used are the following: Machine: Willy Degen UTB 600H Drill type: 8x950 mm, BRK K15 uncoated HSS Depth: 350-400 mm Speed: 60 m/min (2400 rpm) Feed: 0.015 mm/turn (36 mm/min) Coolant: Oil 62.5 bar Ramax LH could be drilled 20m without the need for regrinding of the drill. When drilling in 420 F type at the same hardness, there was a need for regrinding after a ~15 m drilled length. The test was stopped after a 20m drilled length.
Stability Different tests performed at customers have shown an excellent stability during machining operations such as milling and drilling. Customers have noted that this material shows very little movement during their machining operations, which minimizes the amount of shimming and rework that they have to do to get the dimensions “true” to the print requirements. Ramax LH accomplishes this through the use of a higher tempering temperature and a controlled process route during its production, which minimizes the level of stresses found in the completed steel. The chemistry also minimizes the tendency towards higher stresses with an optimized microstructure.
Application Test Results To verify the in house test results given above, a series of customer tests were performed. In all cases, the machining and stability results for this grade indicated that Ramax LH was comparable to or better than the easiest to machine holder stainless grades on the market. As compared to the higher hardness resulfurized grade, milling insert life was extended by three times, cutter widths were increased, and milling speeds were increased by 20 percent. Customer tests including face milling, high feed milling, finish milling, drilling, tapping and boring, all showed favorable results to the grades currently in use for Holder Stainless.
Figure 3: Holder machined in Ramax LH.
Welding Welding is sometimes needed due to design changes or machining errors. Welding will change the structure of the material no matter what precautions are taken. The important factors to consider are how to weld in order to minimize the effect and to use a welding rod adapted to the composition of the material to be welded. The low carbon content of Ramax LH makes the material easy to weld with less risk of hardening cracks during the welding operation than in other 420 F alloy types. Small welds can be welded without the need for preheating. After welding the material, it is still recommended that the weld area be stress tempered in order to reduce the hardness of the weld and the stress level. A 420 weld rod, such as the Stavax TIG rod from Uddeholm can be used when welding might be required.
Corrosion Resistance In addition to good machinability, the steel must be able to demonstrate that it has sufficient corrosion resistance to resist atmospheric corrosion and other attacks common in an injection and extrusion molding environment. An initial test of 14 days in a climate chamber of alternating temperature 8 hours at 35°C (93% humidity) and 8 hours at 15°C, indicated that the results of corrosion on the Ramax LH were equal to if not better than the resulfurized stainless Uddeholm currently provides to the marketplace as shown in the visual evaluation done here versus the modified 420 stainless Ramax HH.
Figure 3: Resulfurized 420 Free Machining Stainless (left) versus Ramax LH after climate chamber testing. Ramax LH shows similar if not better results to this sort of corrosive environment.
Should stress corrosion cracking become a factor in the threaded regions of the steel, if it is used for actual molding applications, its resistance to cracking up to 60 percent of its yield strength, which exceeds the capabilities of other resulfurized stainless alloys.
Conclusion Ramax LH was designed to optimize machinability and stability for the moldmaker, which translates into significant cost savings, reduced setup times and reduced lead times in the delivery of the mold base to the mold user. These factors have been evaluated both on an inhouse basis and through trials at moldmakers and found to show excellent results. Other factors that have been optimized, such as weldability and corrosion resistance, will also mean that this steel can be used in other application areas than holders where these characteristics are also important.
About the Authors: Lena Råhlén has a Master of Science Degree in Material Technology from the Royal Institute of Technology, Stockholm Sweden. She has been employed by Uddeholm Tooling Sweden since 1980 and has worked in the Research and Development Department, in Customer Service Department and in the Marketing Department as Product Manager and now as Application Manager. The latest 15 years has been totally devoted to Materials used in moulds for Plastic applications. Patricia Miller is the Senior Technical Manager with Bohler-Uddeholm. She supports the technical needs of the Bohler-Uddeholm customer base and is a Past Chair of the Moldmaking and Mold Design Division of the Society of Plastics Engineers. She can be contacted via email at pat.miller@bucorp.com or at 1-800-652-2520 ext. 8732.
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