Metal seal Ball Valve sealing surface hardening technique

Metal seal Ball valve sealing surface hardening techniques

First, Overview
High-viscosity fluids in power plants , petrochemical systems , coal chemical industry , with dust mixed fluid and solid granular , highly corrosive fluid medium and so on required the ball valve should be metal seal , so the choice of a suitable metal hard sealing ball valve “ball and seat”  hardening process is very important.

Second, the usual hardening process are the following :

( 1 ) sphere surfacing deposit weld alloy , hardness usually up to 30HRC or more, spheres  surfacing process is complex, low productivity, and a large area of ​​surface welding easy to make parts deformation , as a reslut, currently use of the sphere surface hardening process become less .

( 2 ) the sphere surface hard chrome plating , hardness up to 40 ~ 55HRC, thickness 0.07 ~ 0.10mm, chrome -plated has high hardness , wear resistance , good corrosion resistance and able to maintain long-term bright surface, the process is relatively simple and low cost. But the hard chrome plating hardness will low rapidly by the release of the stress at high temperature. So its working temperature can not exceed 427 ℃. Another disadvantage are  low- chrome plating adhesion , coating fall off easily.

( 3 ) the sphere surface by plasma nitriding , surface hardness up to 50 ~ 55HRC, the nitride layer thickness 0.20 ~ 0.40mm, plasma nitriding hardening process due to the poor corrosion resistance , can not be used in the chemical industry and other fields with strong  corrosion.

( 4 ) the sphere surface supersonic spraying (HVOF) process, the hardness  up to 60 ~ 70HRC, a collection of high- strength, thickness 0.3 ~ 0.4mm, supersonic spraying is the main sphere surface hardening process means . High-viscosity fluids in power plants , petrochemical systems , coal chemical industry ; mixing fluids with dust and particulate solids , highly corrosive fluid medium usually use this hardening process . HVOF oxyfuel combustion process is a high velocity air stream to accelerate the powder particles strike the surface, forming a dense coating process method . During the impact , the faster the speed of the particles (500 ~ 750m / s) and the particle temperature is low (-3000 ℃), so the impact surface , a high bonding strength can be obtained , the low porosity , low oxide content coating.
HVOF features alloy powder particles faster than the speed of sound, or even 2 to 3 times the speed of sound , flow velocity is four times the speed of sound . HVOF is a new process , coating thickness between 0.3 ~ 0.4mm, the coating is mechanically coupled to the workpiece , the combination of high strength (77MPa), coating low porosity ( < 1% ) . The process of the workpiece heating temperature is low (<93 ℃), the workpiece deformation , can be cold spray . When spraying, powder particles high speed (1370m / s), no heat -affected zone , no change in the composition and organization of the workpiece , high coating hardness can be machined .

( 5 ) , a spray thermal spray metal surface treatment process . After the powder which is obtained by a heat source ( metal powder, alloy powder , the ceramic powder may be ) is heated to a molten or plastic state to achieve high , it relies on the ejection stream , a pre- deposited onto the treated surface , forming a surface of the workpiece ( substrate ) with a solid coating ( solder ) layer . Spray and hardfacing alloy hardening process and the substrate are having melting process , carbide and matrix collection at hot melt zone is fully achieved spray surfacing carbide layer performance , to avoid weld heat, melt processing metal contact surface area , the proposed spray surfacing carbide thickness need greater than 3mm .

Third, the hardness collocation of the valve ball surface and seat contact surface

Metal sliding contact surfaces need to have a certain difference in hardness , otherwhise prone to galling . In practice, the difference in hardness between the ball and seat generally in 5 ~ 10HRC, the hardness of the ball can have a better life errand . Due to the complexity of the sphere processing , high processing costs , in order to protect the ball less susceptible to damage and wear , the general choice of the sphere surface hardness greater than the hardness of the seat . There are usually main two collocation of Ball and seat surface hardness are widely used:

① ball surface hardness of 55HRC, seat surface 45HRC, the ball can be supersonic spray Stellite20 alloy surface , seat surface can be surfacing Stellite12 alloy , the hardness of the match is the most widely used for metal sealing ball valve , to meet conventional metal seal ball valve wear requirements ;
② ball surface hardness of 67HRC, seat surface 58HRC, the valve can be supersonic spray tungsten carbide ball surface , seat surface can be supersonic spray Stellite20 alloy , the hardness match are widespread use for coal chemical industry , with high wear resistance and life. In foreign countries , and  the ball seat surface with the same hardness match , the surface of the ball and seat are made of tungsten carbide supersonic spraying process, the surface hardness all are greater than 72HRC, even in the case of higher hardness , the ball and the valve seat surface is also less prone to bite , but in the current domestic production of ball surface,   ball and seat grinding technology with hardness greater than 72HRC are not mature , it is difficult to ensure that the valve ball and seat match perfect, so with less use .
Fourth, hard seal ball valve ball and seat sclerosis attention problems

Metal seal valve ball and seat materials generally choose stainless steel or corrosion-resistant materials , otherwise the carbide seat ( or the ball ) binding layer is easy to corrosion, occurs carbide layer off , reduce ball valve life. Also for different seat ( or the ball ) should choose the right material hardening process , duplex stainless steel materials are widely used in the coal chemical industry , the duplex stainless steel has good resistance to corrosion fatigue and wear resistance. Duplex stainless steel material is a both ferritic and austenitic steels with a structure of ferrite and austenite structure about 50% each , and the second phase is independent of the existence of the organization , and its performance characteristics is both austenitic and ferritic stainless steel features. In ferritic stainless steel features , when the temperature in the range of 400 ~ 500 ℃, prolonged heat will have a strong embrittlement , a phenomenon commonly referred to as 475 ℃ embrittlement ; temperatures in excess of 400 ~ 500 ℃ , the duplex stainless steel performance will be destroyed . The use of duplex stainless steel alloy spray welding process or method , this process occurs carbide and matrix melt melting process ( temperature generally greater than 900 ℃), will destroy the microstructure of duplex stainless steel , so it might not suitable for use with stainless steel spray ( or welding ) carbide hardening process . Duplex stainless steel surface hardening process is suitable for use supersonic spraying process, the hardening process should guarantee duplex stainless steel matrix microstructure will not be damaged .

Fivth, Conclusion

Metal seal ball valve ball and seat using reasonable hardening process , can improve metal seal valve directly which determines the life and performance , reasonable hardening process can reduce manufacturing costs. With the continuous emergence of new technologies , there will certainly be more hardening treatment process , metal seal valves and hardness with the hardening process is a very complex issue , and therefore on the hard seal valve design , material selection, and hardness collocation with the hardening process , should be given full attention.

Leave a Reply