Stellite 21 CoCrMo alloy (cobalt-chromium-molybdenum) is one of the cobalt-based alloys, and it is also a kind of the so-called Stellite alloy. It is a cobalt-based alloy with excellent wear resistance and corrosion resistance. The first cobalt-based alloy was a cobalt-chromium binary alloy, and then it was developed into a cobalt-chromium-tungsten ternary composition, and then later, a cobalt-chromium-molybdenum alloy was developed. Cobalt-chromium-molybdenum alloy is a kind of alloy with cobalt as the main component, containing a considerable amount of chromium, molybdenum and a small amount of nickel, carbon and other alloying elements, and occasionally also containing iron. Depending on the composition of the alloy, they can be made into welding wire, the powder can be used for hard-surface welding, thermal spraying, spray welding and other processes, and can also be made into castings and forgings and powder metallurgy parts.
Cobalt and chromium are the two basic elements of cobalt-based alloys, and adding molybdenum can get finer grains and have higher strength after casting or casting. Cobalt-chromium-molybdenum alloys are basically divided into two categories: one is CoCrMo alloys, which are usually cast products, and the other is CoNiCrMo alloys, which are usually (hot) cast precision machining. Cast CoCrMo alloy has been used in dentistry for decades and is now used to make artificial joints. Cast CoNiCrMo alloy is used to make joints that accept heavy loads such as knee joints and hip joints. However, as a joint implant material, CoCrMo alloy will release Co, Cr, Ni and other harmful ions after being implanted in the human body.
Stellite 21 chemical composition:
Stellite 21 mechanical properties:
According to the current internationally mature experience, the function of the surfacing layer is mainly determined by the chemical composition and dilution rate of the weld metal of the surfacing layer, and the chemical composition of the weld metal depends on the chemical composition of the welding material. When the welding material is selected After that, the chemical composition of the weld metal was basically confirmed. When selecting the welding process, it is necessary to consider preventing external factors from causing changes in the chemical composition of the weld metal or infiltration of other impurity elements; the size of the dilution rate depends on the size of the heat input (E) during welding, that is, heat The larger the input, the higher the dilution rate; otherwise, the decrease, and the heat input is calculated as follows: E=UI/v
In the formula: E is the welding heat input, J/mm; I is the welding current, A; U is the welding voltage, V; u is the welding speed, mm/min. Therefore, when selecting the welding process, it is necessary to ensure the chemical composition of the weld metal of the surfacing layer and reduce the amount of welding heat input. The pre-confirmed process flow path is as follows: blanking→planing and milling-nondestructive testing (PT)→preheating→welding→visual inspection→nondestructive testing (PT)→sample processing→chemical analysis and functional test→material collection→result analysis→ Report clean up and use production (5) 0
Requirements before welding
After the surface of the surfacing welding is confirmed to be free of cracks, pores, interlayer, heavy leather and other surface defects through liquid soaking inspection, clean the surface of the surfacing welding with acetone. For surfacing welding quality materials, preheat the surfacing parts to above 15°C. After the hand touches and feel warm, the welding will be carried out immediately. At the same time, confirm that the relative humidity of the welding environment is not higher than 80%, the wind speed is not higher than 2m/s, and the Ar gas purity is required to reach 99.99%
Requirements during welding
It can be known from the calculation formula that the size of the welding heat input (E) is proportional to the size of the product of the welding current (I) and the welding voltage (U), and the size of the welding speed (v) is inversely proportional. For the non-melting electrode manual argon tungsten arc welding (GTAW) welding method, the welding current is attributed to the preset controllable elements, and the welding voltage and welding speed are attributed to the artificial control and random elements. This is in the fourth part of the French RCC-M specification. Cited in Volume S "Welding". At the same time, it is described in Clause 8.5.4 of ISO 15614-7:2007 "Qualification of Welding Procedures for Metallic Materials Part 7: Overlay Welding": The upper limit of the approved heat input range for each layer is beyond the welding procedure evaluation. The heat input used by the same layer is 25%, and the lower limit is less than 10% of the heat input used by the same layer when the welding process is evaluated.
Because the welding voltage and welding speed are controlled by humans, when selecting welding specification parameters, the priority is to control the size of the welding current. Under the premise of ensuring the quality of surfacing welding, choose a lower welding current value as much as possible, namely Choose "small current, short arc welding, fast, multi-layer multi-pass welding". Its welding specification parameter adjustment. Strictly control the bead spacing during surfacing welding. The next bead should be pressed to half the width of the previous bead to minimize the dilution rate. The welding direction between the weld passes should be welded back and forth one by one to reduce welding stress and deformation. Before welding, the welding power source should be adjusted in advance to the protection state of pre-supplied argon gas and delayed argon-supplied gas. At the beginning, the current should be adjusted on the ignition arc board of the same material, the arc should be ignited, and then the welding should be transferred to the beginning of the welding area to start welding. Starting from the numbered position No. 6 along the central axis of the weld, the weld is welded back and forth to both sides. The crater must be filled when closing the arc to prevent crater cracks from occurring. Welding joints adopt the reflow arc method to ensure the quality of the joints at the welded joints. The welding joints between each weld pass are required to be staggered. The surfacing thickness should be within the range of 3.5~4.0mm. After welding, use rock wool to keep warm and slowly cool to room temperature.
Welding disadvantages and measures
Appearance finishing before and during welding. Thoroughly remove the surface oxide scale, oil stains, impurities, interlayer coatings, molten slag and other harmful impurities, to ensure that the surfacing surface is smooth and lubricated, with metallic luster and no defects such as cracks, pores, slag inclusions, etc. on the surface;
Temperature control during welding, including preheating before welding, temperature control between passes, and slow cooling after welding. Preheating before welding and slow cooling after welding can reduce the cooling rate after welding and reduce part of the residual stress caused by harmful temperature gradient; interlayer temperature control can reduce the high temperature residence time, prevent the coarse crystal embrittlement of the welded joint, and reduce the impact toughness;
Stress relief heat treatment after welding. After the surfacing is completed, the internal restraint stress is large, which simply causes cracking. Through the stress-relieving heat treatment, the internal stress can be eliminated in time to prevent defects from occurring.
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