From the beginning of power feeding and smelting to the complete melting of the charge (except for the solid arch bridge above the melting pool), it is called the charge melting stage. At the beginning of smelting, the specific resistance of the newly added charge is large, the electrode is in direct contact with the charge, and the charge is heated by the resistance heat of the charge. At this time, the input current is small but relatively stable. During this period, the resistance heat is dominant. But this period of time is not long. When the charge under the electrode melts to form three “crucible molten pools”, an arc heat is generated between the electrode and the “crucible molten pool” to heat the charge to gradually expand the molten pool outwards until a communication three electrode is formed The “big molten pool”. During the transition from the “crucible bath” to the “large bath”, due to the reduction of the unmelted charge, the specific resistance gradually decreases, so the charge resistance heat gradually decreases; and the arc heat between the electrode and the “crucible bath” The outgoing ratio gradually increased. After about half an hour from the beginning of melting, arc heat dominates. The above “transition period” is the unstable period of smelting high-titanium slag, one is because the resistance of the circuit (electrode → crucible bath → unchemical charge → crucible bath → electrode) through the current changes with time; the second is “ The solid material above the crucible molten pool often falls into the molten pool and causes a violent reaction to cause the slag to boil, and this “collapse material-slag boiling” phenomenon is irregular.
Titanium alloy determines the proportion of alloying elements according to the following principles:
- (1) The allowable fluctuation range of alloy element and impurity content and the best composition range required for the best performance of the alloy;
- (2) Smelting method and melting times;
- (3) Burning rate and evaporation rate of alloying elements during vacuum wasting smelting;
- (4) The way of adding alloy elements and physical properties.
Generally, for the elements with large burning loss rate and easily volatile, the proportion of ingredients is close to the upper limit or exceeds the upper limit, and the elements that are not easy to lose volatile loss are according to the middle limit of the required range of ingredients. .
B Electrode block pressing
The requirements of consumable smelting electrodes are:
- (1) Sufficient strength;
- (2) sufficient conductivity;
- (3) Straightness;
- (4) The distribution of alloy elements in the electrode is reasonable;
- (5) No moisture and pollution.
The preparation method of monolithic electrode includes pressing (divided pressure and lateral pressure) and extrusion (also divided into horizontal and vertical), the more commonly used is the pressing method.
The density of the electrode block is related to the pressed material. Figure 8-27 shows the relationship between unit pressure and electrode density. Generally speaking, the density of the electrode block is greater than 3.2 g / cm3 to meet the smelting requirements. Generally, presses with a pressure of 300 to 500 MPa are used.
C electrode assembly welding
Electrode assembly welding is to assemble and weld the pressed single electrode block into the electrode with the cross section and length required by consumable arc melting. In industry, argon shielded plasma welding, vacuum plasma welding and electron beam welding are often used. In order to prevent mixing with high specific gravity, tungsten arc welding is generally not used. The purity of argon used for welding is 99.99%.