It is generally believed that thermal fatigue cracking and thermal wear are the two main reasons for the failure of hot work molds. In this regard, the relevant foreign literature is very clear: the damage of the mold and the three mechanisms that limit the increase in the life of the mold are: 1) soldering and chemical erosion damage of liquid metal aluminum. 2) Wear and corrosion. 3) Thermal fatigue cracking. One of them is the most important failure mechanism. They proposed that the use of ferritic nitrocarburizing and ion nitriding can significantly improve the tool life of tool steel. Domestic experiments on aluminum melting loss indicate that when the mold material hardness is 45HRC, the aluminum machining surface loss rate of untreated aluminum is as high as 54.90%. When salt bath sulfur nitrogen carbonitriding is used, the melt loss rate is only 0.10%. After applying salt bath nitrocarburizing (soft nitriding) and adding PVD treatment, the melt loss rate is significantly reduced to 0.10%. It can be seen that the effect of surface modification of H13 steel is very obvious.
Characteristics and development direction of hot work die steel H13
① H13 steel is a hot-working die steel that is widely used in the world. It has high hardenability and resistance to hot cracking. Many applications are used to make aluminum alloy die-casting molds.
② The domestic grade of H13 steel is 4Cr5MoSiVl, and it is now developing in the direction of low Si and high Mo.
③ The surface modification of H13 steel mainly includes the methods of ferrite nitrocarburizing or sulfur nitrocarburizing and physical vapor deposition of hard film. The more superior method is a reasonable combination of these methods.
The surface modification of hot work die steel H13 and aluminum alloy die-casting molds are mainly in the following two aspects:
(1) Ferrite nitrocarburizing and sulfur nitrocarburizing technologies and (2) PVD coating technology. Research papers on these two aspects have been published at home and abroad, but few reports on specific industrial applications. The French HEF Group, which specializes in material surface modification technology, reported examples of industrial applications of H13 steel surface modification in the form of papers at some international conferences. At the same time, HEF Shanghai combined with Shefford The practice of Surface Treatment Technology Co., Ltd. (TS Shanghai) is described with relevant foreign literature (especially the work of NADCA experts and Case Western Reserve University professors).
The best way to solve the surface modification problem of H13 steel is to coat the surface of the mold material with a hard film to prevent it from being wetted by the aluminum alloy melt. HEF Group deposits 3μm thick CERTESS SD coating on the tappet (38CDV5, equivalent to H13 steel) in the die-casting mold of aluminum alloy workpieces for automotive steering operation systems. Its hardness can reach 4000 ～ 4500HV, and the operating temperature can reach 800 ℃ It can also resist the adhesion of aluminum alloy, and its service life is increased to 100,000 times, which is 6 ~ 7 times that of the tappet without deposition treatment.
For how to obtain such a hard film that is not wetted by liquid metal, D. Zhong and J.J. Moore of Colorado School of Mines (CSM) proposed that the structure of the multilayer optimized coating film is:
① First modify the surface of the H13 mold base, such as ferrite nitrocarburizing or ion nitriding;
② 50 ~ 100nm adhesion interlayer such as Ti or Cr;
③ The intermediate graded layer that adjusts the thermal residual stress between the substrate and the coating due to the die-casting operation, which can be determined using finite element simulation methods. They think that it depends on the selected working hardening layer. When the working layer is an Al2O3 layer, the intermediate transition layer is a Ti-Al-N gradient layer;
④ Working coating, non-wetting with liquid metal or glass. For die casting of Al alloy, CrN, TiAlN, TiCB and Al2O3 can be used. The total thickness of the corresponding multilayer structure film is between 5 and 8 μm.
Obtaining excellent quality coatings on mold workpieces by PVD technology should depend on high-performance equipment and process parameters that can be optimally selected. This equipment preferably has the following technical requirements:
① The coating treatment temperature is low;
② Good winding performance;
③ uniform coating deposition;
④ Adopting enhanced ionization rate technology;
⑤ precise coating composition control;
⑥ a certain deposition rate;
⑦Can carry out multi-layer composite coating;
⑧ can get nano-structured coatings;
⑨With PVD and CVD working modes;
⑩Can etch while coating to obtain the best coating quality.
The French HEF Group developed and applied plasma enhanced magnetron sputtering (PEMSTM) technology to precisely control the deposition process of the coating. The highest theoretical density of the coating can be achieved using PEMSTM technology. The HEF Group’s vacuum coating equipment and technology are therefore leading the world. In summary, the main advantages of HEF equipment and technology are:
① The flow and energy of ions can be controlled independently to obtain the coating with the highest density and performance;
② The unique OES system is used to measure the plasma wavelength to achieve precise control of the coating composition;
③ All hard coatings obtained have nano-scale (≤20nm) structure;
④ The uniform bombardment under the environment of low voltage and high vacuum can greatly reduce the sharp angle effect;
⑤ Heating while evacuating to remove moisture sufficiently and quickly to obtain high-quality and efficient deposition effect;
⑥ The minimum cnc machining temperature can reach 80 ° C, which can be used for coating machining on almost all materials, including aluminum alloys and polymer materials;
One device has both PVD and PACVD machining modes, giving customers full support for technology upgrades.
French HEF Group has invested in two series of PVD / PACVD equipment in Shanghai Shefu Surface Treatment Technology Co., Ltd. (Shanghai Songjiang) through Aifu Surface Treatment Technology Co., Ltd. (Shanghai Songjiang). CrN, CrxNY, TiN, TiBN, TiCN, TiALN, and diamond-like DLC films. They use liquid sulfur-nitrocarburizing or liquid nitrogen-carburizing for surface modification of H13 steel substrates before PVD coating. , Referred to as Sursulf / Arcor (Shershef / Arco) technology or Tufftride / Tenifer (provided by Durferrit, its German subsidiary that merged with the HEF Group in 2001).