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  • 04 Nov 2020
    You may need a dental crown for one of many reasons, such as to restore a broken tooth or to bugle head drywall screw manufacturer cover a tooth that has a large filling. A crown is a tooth-shaped cap, placed over your tooth to restore its appearance and strength. It completely encases the visible portion of the tooth. Other reasons it may be needed include protecting a weak tooth from breaking, holding a dental bridge in place, and covering a dental implant. There are different types of dental crowns which may be used. The first type, metal, is usually made of gold alloy or other alloys and can be thinner than other types. For this reason, less of your tooth structure must be removed before the cap can be used. This kind also does not wear down as easily as some other kinds of dental crowns and is not likely to chip or break. Another option is porcelain-fused-to-metal, which is a metal shell fused to a porcelain covering. The porcelain can be made to match the color of your teeth, so this type is more aesthetically appealing. However, the metal part can sometimes show through, appearing as a dark line at or near the gum line. To avoid this, you may opt for an all-porcelain cap. This selection is also ideal if you are allergic to metal. Another type is all-resin, which is less expensive than the other choices. This type is more likely to fracture or wear down than the other types as well.Dental crowns usually require two visits to your dentist. During the first visit, the dentist examines and prepares your tooth. He or she will check for decay or infection to determine whether you need a root canal first. X-rays may be taken to examine the tooth root and surrounding bone. If no other procedures are needed, the dentist will file down the tooth’s chewing surface and sides. If a large portion of your tooth is already missing because of decay or damage, the dentist will use filling material to build it up. Impressions of the tooth and opposing teeth are made and sent to a dental laboratory. The laboratory creates the permanent crown, which usually takes two or three weeks. To protect your tooth during this time, your dentist creates and places a temporary crown. It is commonly acrylic and held in place with temporary cement. Your dentist will likely recommend that you take precautions when eating to avoid dislodging it. For example, you will probably need to avoid sticky foods like caramel and hard foods like raw vegetables. When the dentist receives your permanent cap from the laboratory, you will go back for your second visit. At this appointment, your dentist removes the temporary cap and chec ks the permanent one to make sure it will fit properly. If it is porcelain or porcelain-fused-to-metal, the dentist will also check the color. If the permanent dental crown is acceptable, the dentist will cement it in place.Permanent crowns do not require any special care, but you should still practice good oral hygiene. The underlying tooth is not protected from decay, so be sure to continue brushing and flossing regularly. Dental crowns are usually very sturdy as long as they are properly fitted and placed. If you develop any problems, such as a chipped or loose crown, contact your dentist as soon as possible for care instructions.
    42 Posted by chinafasten ersfactory
22 views Nov 18, 2020
The carbon massfraction of these steels

At the same time, boron can alsoimprove the high temperature strength and creep strength of chipboard screws the heat resistantsteels, improve the red hardness of high speed steel and the cutting ability ofcutter. From this point of view, boron steel not only enhance the overallperformance of the steel, but also reduce the production cost. For nearly 10years, the international steel mills continue to accelerate the development andproduction of boron steel, boron steel’s smelting technology constantly mature,the variety of boron steel is also expanding.The main types of boron steels have alloystructural boron steel, low alloy high strength boron steel and spring boronsteel and so on. Alloy structural boron steel used more often, includingquenched and tempered boron steel, surface treated boron steel and colddeformation boron steel. Quenched and tempered boron steel has high strength,high toughness, high abrasion resistance, can be used in automobiles, tractors,machine tools, mining machinery, power plant equipment.

The carbon massfraction of these steels are more than 0.25%, except added boron separately,also can be added to a variety of alloys such as manganese boron, chromiumboron, manganese-titanium boron, manganese vanadium boron, chromium-manganeseboron and many other alloys; surface treated boron steel is mainly the infiltrationcarbon boron steel, the carbon mass fraction is generally less than 0.25%, thistype of steel with better performance of carburized, the carburized layer doesnot form a large amount of residual austenite, thus have high hardness,abrasion resistance, good fatigue resistance, and small notch sensitivity.Hardenability effect of boron in steel isrelated to the amount of boron, carbon content and the austenitizingtemperature of steel. Generally increases as the carbon content increases, therole of the boron is gradually reduced. In the best austenitizing temperature,the hardenability effect of boron is the best. The studies show that the massfraction of the boron in steel should be about 0.0010%, too low or too high cannot produce the best results.Boron occurs eliguation while the boronsteel solidified, lead to the slab have defects. Some research institutionsfound that the difficulty of producing boron steel in the continuous castingstage. They thought that, even if add a small amount of boron in steel, theslab will also produce surface cracks, internal cracks and center segregation,even appears bleed-out. After added boron, the solidification of the moltensteel in the continuous casting will melt again, by in situ analysis showedthat boron segregated at the grain boundaries and dendrite.

Thus it can be seenthe molten steel maintain the molten state for a long time and prolong clottingtime is the main reason lead to slab defective.Use boron-containing molten iron as rawmaterial produce boron steel can reduce costs. Researchers use boron-containingmolten iron as raw materials to produce boron steel, hot metal through desulphurization,desilication, pretreatment loaded into the converter, adding slagging agent,and then supply for oxygen blowing. According to the requirement of boron steelcomponents, adding silicon, manganese and carbon, and the steel temperature wascontrolled at 1560 ~ 1610,then casting into ingots, and finally obtain boron steel. Some researchersbelieve that use boron-containing molten iron produce boron steel can reducecosts, but while oxygen blowing, the high concentration of oxygen in theconverter will reduce the yield rate of boron, this is the difficulty of use boron-containinghot metal to produce boron steel.In Boron steel production, if not controlwell the craft and boron content will bring many problems. Such as the bad hardenability,leading to the strength, hardness have fluctuations. Although it is possible toadjust the added amount of titanium in the steelmaking to improve theseproblems, but what was worth to noting is that the effect of general alloy elementsimproving the hardenability is increased with its content in the steel, whilethere is an optimum content of boron powder. The moreboron powder content, boron phase along the grain boundary sheet consecutiveprecipitation, it will weaken the toughness of boron steel. Generallyconsidered that 0.0005% to 0.0020% of the mass fraction of the acid solubleboron is the best, can increase the yield of boron in refining, and the addedamount of boron is more stable.Source: