Optimization of Process Parameter in Seat Hardfacing Process for an I C Engine Valves

Optimization of Process Parameter in Seat Hardfacing Process for an I C Engine Valves

Madhu G

Assistant Professor, School of Mechanical Engineering,

Reva University, Bangalore 560064.

Vijee Kumar

2 Assistant Professor, School of Mechanical Engineering,

Reva University, Bangalore 560064.

Robinson P

3 Assistant Professor, School of Mechanical Engineering,

Reva University, Bangalore 560064.

Abstract – This research is carried out to optimize the process parameters in seat hardfacing process used to deposit cobalt based superalloy having higher wear, corrosion and heat resistance in an I C engine valves. The detailed study of operation, equipment & materials are done to find the root causes of the problem. Quality control tools such as pareto diagram, cause & effect diagram are used for the suggestions & modifications to overcome the defects like stellite deposition overflow, head melt, non uniform stellite merging, stellite unfill on seat and blowholes on the seat portion of the I C engine valves.

Key Words: Defects, I C engine valves, Root causes, Seat hardfacing process.

1. INTRODUCTION

   The valve seat of an I C engine valves are repeatedly engaged with valve seat insert and it has to provide proper sealing in the engine cylinder [1]. Especially in an exhaust valve, portion of the valve seat is directly exposed to the high temperature and corrosive products of exhaust gas. Generally the valve seat must withstand exposure to corrosive gases passing there over at high pressure and at high temperatures ranging from about 5380C or higher at different combustion cycle [2].

   Fig 1: I C engine component details.

   Seat hardfacing process to the I C engine valves is one of the well-known processes in the industry. In the manufacture of I C engine valves, it is common practices to weld the valve seat with a corrosion, wear, abrasion and heat resistant alloy to protect the valve seat and enhance the service life of the valve [3].

   The hard facing material used in this process is cobalt based alloy such as stellite® alloy, this is usually applied to the valve seat portion by means of TIG welding process. This study is undertaken in the TIG welding machine, which is used for seat hardfacing process. During seat hardfacing process it is observed that defects like stellite deposition overflow, head melt, non-uniform stellite merging, deposition unfill on seat and blowholes. These defects increased the rejection level to the top position due to variation in the process parameters.

   This study aim is to optimize the process parameter to reduce the rejection level by detailed study of operation, equipment, and materials by evolving suitable counter measure to get the defects free products in the seat hardfacing process.

2. HARDFACING PROCESS

   The hardfacing process is a technique, which involves depositing a layer of hard material like, cobalt based alloy to an I C engine valve for the purpose of increasing the required property. The use of this technique has increased significantly over the year as industry has come to recognize that base materials of softer, lower cost material can be hardfaced to have the same wear and corrosion resistance characteristics as more expensive base material of a harder material [4].

   The selection of this process is mainly depended on a number of factors like, nature of work to be hardfaced, function of the component, base metal composition, size and shape of the component, accessibility of weld equipment, number of same or similar item to be hardfaced etc [5].

   1. I C engine valve material and hardfacing alloy

      I C engine valve material:-The detailed chemical composition and mechanical properties of the I C engine valve material of 21-4N used in this process is listed in the below Table 1&2 respectively.

      Table 1: Chemical composition of the I C engine valve material 21-4N.

      Chemical Composition of 21-4N Raw Material
      C%	Si%	Mn%	P%
      0.48/0.58	0.25 Max	8.00/10.00	0.04 Max
      S%	Cr%	Ni%
      0.03 Max	20.0/22.0	3.25/4.50

      Table 2: Mechanical properties of I C engine valve material.

      Mechanical Properties
      Raw materi al	Hardne ss	Yield strength Kg/mm2	Tensile strength Kg/mm2	Elongat ion %	Reduction area %
      21-4N	25 HRC Min	57 Min	90 Min	8 Min	15 Min

      Hardfacing alloy:- The chemical composition and hardness of the hardfacing material used in this process are listed below.

      Table 3: Chemical composition and hardness of the hardfacing alloy.

      Stellite grade	HRC	C%	Mn%	Si%	P%	S%
      Stellite®1	54	2.43	0.3	1.2	0.009	0.007
      	Cr%	Ni%	Mo%	W%	Fe%	Co%
      	30.5	1.8	0.3	12.7	2	Bal
      Stellite®6	HRC	C%	Mn%	Si%	P%	S%
      	43	1.25	0.08	1.26	<0.001	0.013
      	Cr%	Ni%	Mo%	W%	Fe%	Co%
      	30.39	2.55	0.02	4.82	2.48	Bal
      Stellite®1 2	HRC	C%	Mn%	Si%	P%	S%
      	50	1.51	0.24	1.37	0.005	0.015
      	Cr%	Ni%	Mo%	W%	Fe%	Co%
      	28.5	1.96	0.13	8	1.8	Bal

   2. Component processing details.

   Before seat hardfacing process of an engine valves following operations are carried out to obtain required size and shape of the engine valves.

   Raw Material

   Seat Hardfacing

   Raw Material

   Seat Hardfacing

   Bar Cut

   Seat Recess Forming

   Bar Cut

   Seat Recess Forming

   Bar

   Grinding

   Turn Head

   Bar

   Grinding

   Turn Head

   Upsetting & Forging

   Heat Treatment

   Upsetting & Forging

   Heat Treatment

   Fig 2: The detailed process flow diagram.

   Fig 3: The schematic process flow diagram.

3. TIG WELDING PROCESS FOR SEAT HARDFACING

   In this process the electric arc fuse the hardfacing alloy on a seat portion of the I C engine valve in order to improve the required properties. During the process, the hardfacing alloy is fed into a welding zone generated by the TIG arc at high temperature. All welding parameters, including hardfacing alloy feed rate, power input, shielding gas flow rate as well as torch and workpiece movement are automatized by the TIG welding machine, which is shown in the below figure 4.

   Fig 4(a) TIG Welding machine

   Fig 4(b) Partial schematic diagram for seat hardfacing process.

4. DEFECTS AND PROBLEMS IDENTIFICATION

   1. Defects identification

      Fig (a) Stellite deposit overflow

      Fig (b) Head melt

      Fig (c) Deposit unfill on seat

      Fig (d) Non uniform seat merging

      Fig (e) Blow holes

      Fig 5: The Seat hardfacing process defects images.

   2. Problems identification

   The seat hardfacing process is carried out with process parameters. Even though the seat hardfacing was satisfactory with these parameters, the results were not consistent. The rejection in this process was found to be around 2549 PPM. This was analyzed with the help of QC tools like pareto diagram shown in below figure 6.

   Seat hard facing Process Rejection Pareto April to July- 2012

   Seat hard facing Process Rejection Pareto April to July- 2012

   Defects

   Defects

   1400

   1200

   1000

   800

   600

   400

   200

   0

   1400

   1200

   1000

   800

   600

   400

   200

   0

   1280

   1280

   1050

   1050

   121

   121

   Stellite Deposit Stellite

   Stellite Deposit Stellite

   Blow holes

   Blow holes

   52

   52

   46

   46

   Deposit Head Melt – unfill on Over Flow –

   DS

   Deposit Head Melt – unfill on Over Flow –

   DS

   DS

   DS

   seat – DS

   seat – DS

   Stellite merging not ok – DS

   Stellite merging not ok – DS

   PPM

   PPM

   Fig 6: Total forge shop rejection Pareto diagram

5. DETAILS OF INVESTIGATION

   1. Identification of the defect

      The defects are identified at the time of final inspection and also after the seat hardfacing process of I C engine valve. Cause and effect diagram is used to identify the possible causes for the defects.

      Cause-and-Effect Diagram for the rejection of I C Engine Valve

      Cause-and-Effect Diagram for the rejection of I C Engine Valve

      Material

      Man

      Material

      Man

      Head Dia Variation Head Not Clear

      Seat Recess Form Not Clear

      Chill Inside Dia Variation

      Improper Nozzle Angle Setting

      Tungsten Electrode tip grinding Frequency Unskilled Operator

      Head Dia Variation Head Not Clear

      Seat Recess Form Not Clear

      Chill Inside Dia Variation

      Improper Nozzle Angle Setting

      Tungsten Electrode tip grinding Frequency Unskilled Operator

      Chill Run Out More

      Seat Recess Run Out More Head Run Out

      Face Run Out More

      Chill Run Out More

      Seat Recess Run Out More Head Run Out

      Face Run Out More

      Methods

      Methods

      Insufficient Cooling Of The Chill Improper Oscillation Of The Nozzle

      Stellite Rod Feed Variation Current Variation

      Chill Rotation Speed Variation

      Machines

      Insufficient Cooling Of The Chill Improper Oscillation Of The Nozzle

      Stellite Rod Feed Variation Current Variation

      Chill Rotation Speed Variation

      Machines

      Rejection of I C Engine Valve

      Rejection of I C Engine Valve

      Fig 7: Cause and effect diagram for the rejection of I C engine valve

      Based on the above identification the following possible causes are found significant and all these cause are validated by simulating the defects are shown in the below table 4.

      Table 4: Significant causes for the rejection of seat hardfacing process.

      SL NO	DEFECTS	CAUSES
      1	Deposition overflow, stellite unfill on seat and non – uniform merging.	Chill rotation speed variation.
      2	Head melt, stellite unfill on seat, non-uniform merging and blowholes.	Current variation.
      3	Deposition overflow, stellite unfill on seat.	Stellite rod feed variation.
      4	Head melt, Blow holes.	Seat recess form not clear.
      5	Deposition overflow, stellite unfill on seat and non – uniform merging.	Coining temperature variation.
      6	Deposition overflow, stellite unfill on seat and non – uniform merging.	Forging pressure variation.
      7	Head melt.	Head dia variation.
      8	Blow holes.	Shielding variation.	gas	flow	rate
      9	Blowholes, stellite unfill on seat.	Tungsten tip grinding method and frequency.
      10	Deposition overflow, stellite unfill on seat and head melt.	Improper nozzle & stellite rod feed angle setting.

   2. Experimentation

      Process Variables	Actual	Recommended	Effects
      Chill rotation speed high	20 Sec	28 Sec	By increasing Chill rotation time provides more time for depositing required amount the Stellite alloy and leads to avoid the Stellite unfill on seat.
      Chill rotation speed low	35 Sec	28 Sec	By decreasing the Chill rotation time for avoiding more amounts Stellite alloy deposition leads control the Stellite deposition overflow.
      Initial current high	320 Amp	280 Amp	By setting the Initial current to the actual specification can be possible avoiding the Head melt.
      Initial current low	220 Amp	280 Amp	By setting the Initial current to the actual specification can be possible avoid the Non uniform Stellite merging.
      Stellite rod feed high	200mm	180mm	By setting the Stellite rod feed to the actual specification leads to control the Stellite deposition overflow.
      Stellite rod feed low	150mm	180mm	By setting the Stellite rod feed to the actual specification leads to control the Stellite unfill on seat.

      Process Variables	Actual	Recommended	Effects
      Chill rotation speed high	20 Sec	28 Sec	By increasing Chill rotation time provides more time for depositing required amount the Stellite alloy and leads to avoid the Stellite unfill on seat.
      Chill rotation speed low	35 Sec	28 Sec	By decreasing the Chill rotation time for avoiding more amounts Stellite alloy deposition leads control the Stellite deposition overflow.
      Initial current high	320 Amp	280 Amp	By setting the Initial current to the actual specification can be possible avoiding the Head melt.
      Initial current low	220 Amp	280 Amp	By setting the Initial current to the actual specification can be possible avoid the Non uniform Stellite merging.
      Stellite rod feed high	200mm	180mm	By setting the Stellite rod feed to the actual specification leads to control the Stellite deposition overflow.
      Stellite rod feed low	150mm	180mm	By setting the Stellite rod feed to the actual specification leads to control the Stellite unfill on seat.

      The table 5 summarizes the changes effected in various process parameters and their effects.

      Seat recess form not clear	5 nos out of 250	Nil	By providing clear Seat recess form can be possible to control the Head melt, Stellite deposition overflow.
      Head dia high	58.98m m	58.85mm	By providing actual specification can be possible to control the Head melt.
      Head dia low	58.72m m	58.85mm	By providing actual specification can be possible to control the Head melt and Stellite deposition overflow.
      			By adopting Flow meter
      			can be easily set to the
      			required Shielding gas
      			flow rate and this leads to
      			avoiding Blowholes. And
      Shielding gas	No Flow	Suggestion to adopt	by using nozzle with gas
      flow rate	meter	Flow meter.	lens can be possible get
      			the flow of shielding more
      			stable at a longer distance
      			thereby can be achieve
      			more shielding at the
      			welding zone.
      			By adopting Pyrometer
      			for measuring Coining
      Coining temperature high	No Pyromet er	Suggestion to adopt Pyrometer	temperature can be easily set the required temperature, if it is set to more than required causes
      			to defects like Deposition
      			overflow and Non
      			uniform Stellite merging.
      			By adopting Pyrometer
      			for measuring Coining
      			temperature can be easily
      Coining temperature low	No Pyromet er	Suggestion to adopt Pyrometer	set the required temperature, if it is set to less than required causes
      			to defects like Deposit
      			unfill on seat and Non
      			uniform Stellite merging
      			By using specific tungsten
      			grinding machine can be
      			possible to achieve 500
      Tungsten tip		Suggestion use	nos and it is possible to
      grinding	150 nos	tungsten grinding	avoid Blowholes due to
      frequency		machine	contamination and Stellite
      			unfill on seat due to burn
      			out of the tungsten tip.
      			By setting the Nozzle and
      			Stellite rod feed angle to a
      Improper nozzle and Stellite rod feed angle setting	No Angle plate	Suggestion to fix the angle plate	600-750 and 150-300 respectively can be possible to control the Stellite deposition overflow, Head melt and Non uniform Stellite
      			merging.

      Seat recess form not clear	5 nos out of 250	Nil	By providing clear Seat recess form can be possible to control the Head melt, Stellite deposition overflow.
      Head dia high	58.98m m	58.85mm	By providing actual specification can be possible to control the Head melt.
      Head dia low	58.72m m	58.85mm	By providing actual specification can be possible to control the Head melt and Stellite deposition overflow.
      			By adopting Flow meter
      			can be easily set to the
      			required Shielding gas
      			flow rate and this leads to
      			avoiding Blowholes. And
      Shielding gas	No Flow	Suggestion to adopt	by using nozzle with gas
      flow rate	meter	Flow meter.	lens can be possible get
      			the flow of shielding more
      			stable at a longer distance
      			thereby can be achieve
      			more shielding at the
      			welding zone.
      			By adopting Pyrometer
      			for measuring Coining
      Coining temperature high	No Pyromet er	Suggestion to adopt Pyrometer	temperature can be easily set the required temperature, if it is set to more than required causes
      			to defects like Deposition
      			overflow and Non
      			uniform Stellite merging.
      			By adopting Pyrometer
      			for measuring Coining
      			temperature can be easily
      Coining temperature low	No Pyromet er	Suggestion to adopt Pyrometer	set the required temperature, if it is set to less than required causes
      			to defects like Deposit
      			unfill on seat and Non
      			uniform Stellite merging
      			By using specific tungsten
      			grinding machine can be
      			possible to achieve 500
      Tungsten tip		Suggestion use	nos and it is possible to
      grinding	150 nos	tungsten grinding	avoid Blowholes due to
      frequency		machine	contamination and Stellite
      			unfill on seat due to burn
      			out of the tungsten tip.
      			By setting the Nozzle and
      			Stellite rod feed angle to a
      Improper nozzle and Stellite rod feed angle setting	No Angle plate	Suggestion to fix the angle plate	600-750 and 150-300 respectively can be possible to control the Stellite deposition overflow, Head melt and Non uniform Stellite
      		merging.

6. RESULT AND DISCUSSION

   A few trails were conducted to ensure the consistency in the results with a modified process parameters and adopting new suggestions for this process, they were found to be meeting the quality requirements.

   Recommended for the shielding gas flow meter to control flow rate of the shielding gas. This gives fine control shielding gas flow rate and provides to avoid more shielding gas consumption from 10 liter/min to 6 liter/min.

   Suggestion is made to use ceramic nozzle with gas lens instead of ceramic nozzle. This requires less amount of shielding gas and helps to make the flow of shielding gas more stable at a longer distance.

   Recommended for tungsten grinding machine to prepare the tungsten electrode tip. This gives less problems with arc starting, arc stability, less contamination and leads to long life.

   The procedure is given to set the stellite rod feed angle to 15-300 and TIG torch angle to 60-750. This avoids melting of valve head, stellite deposition overflow.

   Suggested to preheat the stellite deposition temperature between the ranges of 400-4500C with the help of Pyrometer. This temperature is convenient for getting defects free product.

7. CONCLUSION

Recommendation was made for process parameters and it was successfully implemented. New suggestions have saved Rs 1, 12,835 appx to Rs 53,445 appx and also reduce the rejection 2549 PPM to 883 PPM.

ACKNOWLEDGMENT

The author wish to thank the management of M/s Kar mobiles Ltd, Tumkur, Dr. R Suresh Associate professor, Department of Mechanical Engineering, SIT, Tumkur and Mr. R Arumugam Quality manager, Kar mobiles Ltd, Tumkur.

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