Analysis of influence factors on molybdenum wires quality and measures of technology improvements – Part II

Hi, here is TAGUTI news. Can you remember the last topic that we haven’t finished yet? Yes, influence factors on molybdenum wires’ quality and measures of technology improvements. Today we’re gonna continue telling about main factors that influence molybdenum wires’ quality and how to improve molybdenum wires quality.

*Influence of processing status
Whichever technology has been chosen to produce molybdenum wire, it can be concluded to three basic processing methods: rolling, rotary swaging and stretching. Though the blank is in state of three dimensional compressive stree while rolling and rotary swaging, because roll force is much stronger than rotary swaging force, it makes pressure stress strong and uniform while rolling, so it is more beneficial for plastic deformation. While stretching, the wire material is effeted by pulling and tensile stress, so the molybdenum wire is easily broken. However, since the processing way of rolling and rotary swaging have limits(mainly used for cogging and rough machining due to structural features of equipments) and there are requirements of wire specification, stretching is the obbligato method of finish machining for molybdenum wire production(especially for thin molybdenum wire). During multi-pass deformation processing, it is necessary to analyse molybdenum wire’s productive appearance according to its compositional characteristics, such as mechanical property, actual force situation, size range and specification etc. In order to eliminate work hardening of molybdenum wire, it is important to properly allocate working rate for multi-pass deformation processing and set up an annealing point. So that mechanical property of molybdenum wire can be improved, production efficiency can be increased and the product quality can be guaranteed.

Processing measure for improvement of molybdenum wire quality
*Tighten the screws of raw material composition and particle size
The most important part to ensure molybdenum wire quality is strictly control the purity and impurity content of molybdenum powder. Such as painting molybdenum wire, it is supposed to increase the Mo content from 99.93% into 99.95% or even more than this percent, and keep the other impurity element content(C, N, O, P, S etc.) equal to or less than 0.005%. Such as high temperature molybdenum wire, it should prevent added elements from segregation. Because the molybdenum wire is easily broken when the content of element K and Si exceeds the standard, so it is necessary to add moderate element Ca to improve the tensile strength and brittleness resistance of molybdenum wire. After adding element La, Y, Ce and rare earth, It could make special molybdenum wire’s recrystallization temperature increase 400 to 600 degree centigrade. And if molybdenum powder can be increased appropriately and the period of mixing material can be extended, the uniformity of component distribution can be improved, especially the beneficiation of impurities can be decreased. However the time of mixing material should not be too long, in order to avoid the oxidation of powder material and the polution of element Fe, Ni to powder material. If conditions allow, it is better to protect by inert gas when mixing the material.

Generally speaking, it is suitable to keep the average particle size of molybdenum powder around 3.6 – 4.1μm. And for thin molybdenum wire(e.g. d0.127-1.59mm), it is supposed to pick superfine molybdenum powder, whose particle size is around 1 – 3 μm. The apparent density is about 0.98 – 1.10g/cm³. It is ought to adjust the rate of temperature increase and soaking time in every sintering part according to particle size of molybdenum powder and apparent density. It is beneficial for volatilization of impurities. So that the density and the grain size of sintering molybdenum rod can reach the desired value.

*The choice of process route and the adjustment of machining parameter
Because rolling cogging can make molybdenum rod material have excellent mechanical property and stable processing property. So it had better give priority to choose this way of cogging. Especially when producing one whole painting molybdenum wire, which has no welding spot and has a bit heavy piece weight(e.g. More than or equal to 10kg), rolling is even more indispensable part in processing. During manufacturing molybdenum wire, multi units of tandem rolling and precision rolling can even be used, without rotary swaging, directly rolled into the drawbench with bigh wheel to stretch. Even to electric lighting molybdenum wire, grid molybdenum filament with high precision requirements and wire cut edm molybdenum wire, it is better to stretch the molybdenum wire after rolling cogging. It’s much more economical to achieve high quality products with high uniformity and stable property.

While rolling cogging, the temperature should be kept around 1400 degree centigrates. And the temperature can be decreased to 1150±50 degree centigrates in the subsequent rolling process. The compression ratio of rolling passes should be 20 to 25 percent. But when crossing the two racks in the beginning and the end of tandem mill units, the compression ratio should be controlled as 15 percent by considering of plastic. Because the deformation degree and the speed(2.5~6.0 m/min) of rotary swaging is low, its heating temperature range is a little wider than rolling temperature, and the working rate of rolling passes should be kept about 15%±3%(The minimum value could be taken before making round wire by rolling, after making round wire by rolling, the maximum value could be taken.) In the stage of stretching, the working rate of rolling passes should be adjusted according to the specific conditions of rod and wire material. For example, the working rate of rolling passes can reach 30%~40% while rough work of rolling rod with d6.0mm, and the working rate of rolling passes should be controled by 20%~25% while first stretching of d3.0mm rotary swaging rod material. The working rate of rolling passes should be decreased step by step to 10% following the diminution of wire diameter.

During stretching process of molybdenum wire, annealing process can be used to remove work hardening, reduce the deformation resistance, so that plasticity can be increased and processability can be improved.

Generally speaking, when diameter of finished molybdenum wire less than 1.0mm, it is necessary to do intermediate annealing in the process of stretching. And the setting up of annealing point is directly related to the compositional characteristics of molybdenum wire and the percentage reduction of area in the process of stretching. Like pure molybdenum wire, it is better to do annealing when the stretching percentage reduction of area is 91%~95%. If the original diameter of molybdenum wire is big while stretching, for control of annealing temperature and time, it is necessary to think about the work hardening capacity of wire material before annealing and the wire diameter by annealing point. For example, the annealing temperature of pure molybdenum wire should be controlled by 900~1100℃, the annealing time should be 15~25min. When the situation of work hardening is serious, the diameter of wire material is big, the annealing parameter should take a higher value, otherwise it should take a lower value. For high temperature and special molybdenum wire, which have high recrystallization temperature, its annealing temperature should be increased to 1200~1750℃ to improve the grain structure and the processability of molybdenum wire.

There will be a few defects in the process of rolling rod material, like scratch, pitted surface, crack, flash etc. It will cause quality problem of molybdenum wire in the subsequent process of long line stretching, such as burr, flashing, craze or fracture. It is necessary to take effective measures to solve a series of problems, for example, alkali wash with annealing. The molybdenum wire will enter into the alkaline bath filled with nitrate of potash and a little sodium hydroxide through the wire feeding system. It makes the molybdenum wire react with the heated molten alkali in the alkaline bath. Then the alkaline-washed molybdneum wire should be annealled after cleaning and soaking by water. It can not only remove the surface defect of molybdenum wire, but also convenient for lubricant and aquadag adhering to the surface of molybdenum wire, it provides favorable conditions for the subsequent stretching process. The average spared quantity is about 3%~4%, the deviation is less than 1%. The alkaline degree can be controlled by adjusting the wire moving speed, the temperature and concentration of alkali liquor.

Usually after electrolytic cleaning and high temperature straightening processing, it can achieve high quality straightening white molybdenum wire. E.g. Put d0.121mm molybdenum wire into the electrolyte of NaOH, which has concentration of 10% and temperature of 15~30℃. After electrolytic cleaning in the condition by electric current of 9A, voltage of 15V, rewinding speed of 8m/min, then heat and straighten under the hydrogen of 1200℃. It will achieve straightening white molybdenum wire with d0.117mm, which has bright and smoothing surface, uniform wire diameter, high strength, good high temperature property, glancing flatness and toughness. It can meet requirements of automatic lamp equipment in the field of high quality electric light source.

In conclusion, molybdenum wires quality is related with raw material, technology and so many other factors. On the basis of strengthening control by molybdenum powder elements and particle size, it’s important to select reasonable technological route for the situation, and it’s necessary to set up and adjust the processability of rolling passes, the annealing point and the annealing parameters.