Hard Facing on Ball Valve by Automated PTA System

HARDFACING ON BALL VALVES BY AUTOMATED PTA SYSTEM

• Hard-facing, synonymous with surface engineering for metals is well established process and procedure for many decades. Since hard facing is a very vast subject which begins at requirement and end with service life or failure analysis, this article focuses on deposition of hard facing material by welding process and in particular by PTA.
• Deposits of hard facing material has been very popular with conventional welding processes like Arc welding (SMAW), Gas Tungsten Arc welding (TIG), Gas Metal Arc welding (MIG/MAG). The popularity to these processes is largely contributed because of the simplicity of the process and the large range of consumables available. However with new challenges being faced by the industry massive research has been done on the new processes and consumables.
• Plasma Transferred Arc (PTA) is one such process which although known to the industry for past more than 25 years, the limitation was the high capital cost and limited range of consumables. The conventional processes are mostly manual or automated, the PTA is necessarily a automated process.
• Advances in the field of Electrical, Electronics, Metallurgy has made this process affordable to the industries in the field of hard-facing.

WHAT IS PTA ?

• An extension to plasma welding when used for hard-facing or cladding using powder as a filler material is known as Powder Plasma welding. Also commercially known as PTA i.e. Plasma Transferred Arc welding. Being a welding process the advantages of the PTA are enormous compared to other Flame or Arc spraying processes. Also as compared to the commonly available welding methods  the PTA scores well above processes like SMAW, TIG, MIG.
• However all the advantages come at a cost in form of “Automation”. PTA is a process which is not meant to be used manually and call for an automation is to certain level mandatory.
• The level & topology of the logic for the control of automation may differ from manufacturer to manufacturer however the automation can be broadly divided in 3 categories,

1. Semi-automated with timers
2. Numeric controlled (NC) with micro controller, alpha numeric display, keypad
3. Fully computer based CNC system

• The basic differences can be judged from the working capacity, the flexibility of the operation, repeatability of the job, skill level of the operator etc.
• As the subject of automation is very vast and beyond the scope the article, only the insight into high end automation has been focused here in next few pages.
• The PTA process has multiple set of parameters to be handled which can be broadly grouped into gas, mass, motion & power. The parameters covered by each group are as under.

Gas

• Plasma gas flow & pressure
• Shielding gas flow & pressure
• Carrier or Powder gas flow & pressure

Mass

• Powder feed rate
• Powder feed down slope

Motion

• Surface speed of the job
• Travel speed of the torch
• Torch Oscillation

Power

• Welding current
• Arc gap

Although the conventional method of handling certain parameters like gas, powder flow, welding current can be handles manually but it calls for the skill & the experience of the welder / operator.

Exploiting the advantage of the CNC topology, practically all the parameters involved in the process can be programmed and by allowing the computer to handle the process the need for the skilled welding or operator can almost be eliminated. As in any welding qualification the initial setting of parameters can be done under the expert’s guidance.  Once set the operator job is only limited to the loading & unloading of the job on the system. Its is to be noted that any pre or post weld process required to achieve the desired result have to completed in any case.

A typical CNC based system can be wired around high grade of components which include.

• Industrial mother board
• High speed PLC configurable with external add on modules.
• High resolution graphic display with Touch Screen facility.

Other supporting devices like interlocks, sensors, measuring devices are to be compatible with the CNC configuration.

To achieve the desired result, the compatible mechanical system with high degree of accuracy is required typically employing Ball screw mechanism, planetary gear boxes  etc. is a must to squeeze maximum benefits out of CNC system.

A typical CNC base PTA system looks as shown in figure1

Figure - 1: Typical CNC based PTA System

With the necessary support of interfacing devices for signaling, measurement, control & correction a complete fail safe PTA CNC base system can control literally all the parameters involved in the process which are listed as under

• Plasma gas flow & pressure
• Shield gas flow & pressure
• Powder or Carrier gas flow & pressure
• Gas pre flow time
• Gas post flow time
• Welding current
• Welding voltage / Arc gap
• Rotating speed of the job
• Linear travel speed of the torch
•  Weld position of the torch
• Torch oscillation width]
• Torch oscillation speed
• Dwell time
• Weld current down slope
• Powder flow down slope

A typical control screen on CNC display shall look like as in figure 2 & 3

Figure - 2: Display Screen for Welding Current

Figure - 3: Display Screen for Motion Control

Other than the control of parameters the system employs typically single touch operation which controls the sequential flow of operation automatically which is as under.

• Sense the Park or No Weld position of the torch
• Bring the torch to Weld position above the job
• Initiate the gas
• Initiate the pilot arc
• Start the welding arc
• Start the job motion
• Start the torch motion
• Start the oscillation
• Start the powder flow
• Deposit desired bands of weld
• Deposit desired passes of weld
• Bring the weld current and powder flow to down slope mode at the end of the job.
• Stop welding & all the other motions
• Allow desired post flow of gas.
• Bring the torch back to park position     

Figure - 4: Axis Jobs

Engine Valve

Valve Plug

• Many components for hard-facing are circular in shape like seat ring, engine valve, valve seat, plugs etc. which are mostly done on 2-axis system. While hard-facing these types of components two motions are prominent.
• First is job in circular motion and second is boom in linear. It is required to understand that the torch is mounted on the boom hence there is relative motion between the job and the torch. However there are two ways to achieve the relativity.

Step-Over method

• In this case job rotates full 360 degree or additional overlap degrees as per requirement. Once the single rotation is complete the rotation stops and the linear motion of the torch starts. This is called step-over. Once the set linear motion is complete the rotation starts again. Single rotation and single linear motion is called cycle. In CNC system the degree of rotation can be set up to 360 or above or below 360 as per requirement. The boom travel i.e. linear travel can be set in millimeters. Number of circular deposits are generally called as bands.

Step Over Method

Spiral method 

• In this case the circular and linear motion work simultaneously and continuously. This gives the deposits in spiral shape.
• An important issue is to be addressed here which relates to the circular motion. If the job is held in horizontal position (refer valve plug fig.4), irrespective of the diameter of the job the speed of rotation remains constant throughout the job. But if the job is placed horizontally the diameter of welding increases from inside - out or decreases from outside – in. In such case the speed of rotation is also required to be reduced or increased respectively to meet the welding speed.

Spriral Method

• CNC system takes care of all these issues and allows to program, monitor and modify the parameters related to the rotation speed, boom speed, number of bands, and change in rotation speed as per the bands if required.

Deposition on circular 3-D jobs.

• However one job i.e. Ball of the ball valve is odd one out. Just idea of coating a full surface of the ball will make one imagine the complexities involved. And when the job is actually tried for welding, ordinary system will simply fail to the challenges involved.
• Typical ball is shown in figure 5.

Ball Valve cladding with ER CrNiMo -3 

• However the automation developed by Arcraft is so well designed that it can meet all the challenges involved in surfacing of the ball. It is typically a 3-Axis system but just being that does not make system suitable for the application. A well designed software considers all the parameters involved in the process. 2 axis as required in banding method are also required in 3 axis method. However the third axis is Tilting. The rotary table on which the job is held during welding also has the facility to tilt the job. However just manual tilting may not be of any use in case of 3-axis mode. The tilting is also required to be co-ordinated with circular and linear motion. It is the combination of circular banding mode with co-ordinated tilting which helps cover the entire surface of the ball for depositing. 
• Also the ball has a passage or bore to pass the medium which the valve is made for, it is more of a hollow shape, due to which continuous welding on the surface is impossible. In such case the system senses when the edge of the hollow path is reached while rotating the weld stops automatically. However the keeps rotating and the system sense the next edge of the hollow path where again the weld starts automatically. This way the entire ball surface can be deposited with single setting.
• By allowing the CNC to virtually control all the activity of the process there is an endless list of advantages of such system

Saving in Time

• The time saved is what otherwise would be lost in activities like
• Adjust the over the job every time the new job is placed
• Manually raise the torch every time the job is finished
• Manually adjust the powder flow rate for every new job.
• Manually adjust the current for every new job
• Manually adjust the speed of job, torch, oscillation of every new job.

Uniformity of weld deposit

• Highly uniform weld deposit
• No crater at the end of the weld
• Optimum gas flow ensure no blow holes and no oxidation of the job.
• Highly repeatable results

Saving in powder

• Volumetric control ensures optimized flow rate.
• Minimal  wastage of powder.

No dependency on skilled labor

• Machine is used by Operator not Welder.
• Increase in the rate of production.

The bottom line

• As required by any industry is fulfilled in the form of
• Reduced cost of deposit per job or per square inch
• Reduced cost of labor
• Reduced cycle time hence increase in the production rate
• Very low rate of rejection
• Reduced inventory of powder & gases.
• As general conception goes that CNC machines demand high capital investment does not really hold ground here in the case of CNC based PTA machine.
• Carefully designed with optimized use of high quality of raw material, choosing the right option for the electronics involved can make a highly efficiency system at much affordable price.

Visit us: www.arcraftplasma.com                                             Email us: salesarcraft@gmail.com

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