SASTECH Journal 38 Volume 11, Issue 2, Sep 2012

Improving Productivity in a Heat Treatment Shop for Piston Pins

Sandeep K.1, Ajit Kumar 2, Mahesh N. S.3 1- M. Sc. [Engg.] Student, 2- Professor, 3- Head of Dept.

Mechanical and Manufacturing Engineering Dept. M. S. Ramaiah School of Advanced Studies, Bangalore 560 058

Abstract

Productivity improvement is a challenge to every industry and it means efficiency improvement at all stages i.e. man power, energy, machinery, process, money etc. Productivity is one of the major factor contributing to the profit of any company. The study focuses on improving the productivity of piston pin heat treatment process with continuous improvement and process improvement tools such as process mapping, FMEA’s, control plans, and statistical techniques to increase productivity and create bottom-line savings.

The study aims to achieve the optimum utilisation of sealed quench furnace capacity by improving the fixture design, optimizing the lot size, improving the process parameters and thus improving the productivity of the heat-treatment process. The fundamental tools for improving a process is done by identification of root causes with the help of a fish bone diagram and why-why analysis. Furthermore FMEA identifies potential product related process failure modes and prevents wastes, rework, reduce cycle time and cost. The proposal or solution for the material and design selection procedure was obtained with the help of literature review finding, prioritization of various solutions by using FEA analysis and CES.

The ultimate results were improvement in productivity for selected five elements is 21.84%. The improvement of fixture design and wire holding system of parts helped to eliminate 14% of fixture weight to hold more parts. Effort has been made to improve the fixture design to hold more parts and FEA analysis is done to check the design behavior under static and thermal load. The productivity improvement resulted in a cost benefit of Rs. 10,00,000 per month.We conclude that the study has led to significant improvements in process efficiency.

Keywords: FMEA, Fixture design, Heat treatment, FEA

Abbreviations FEA Finite Element Analysis PLC Programmable Logic Control SOP Standard Operating Procedure SQF Sealed Quench Furnace

1. INTRODUCTION

Automatic transmission helps in attaining better control of Case hardening produces a hard, wear-resistant surface or case over a strong, tough core. The principal forms of casehardening are carburizing, cyaniding, and nitriding. Only ferrous metals are case-hardened. Case hardening is ideal for parts that require a wear-resistant surface and must be tough enough internally to withstand heavy loading. The steels best suited for case hardening are the low-carbon and low-alloy series. In case hardening, change occurs at the surface of the metal chemically by introducing a high carbide or nitride content. The core remains chemically unaffected. When heat-treated, the high-carbon surface responds to hardening, and the core toughens.

Carburizing- Carburizing is a case-hardening process by which carbon is added to the surface of low-carbon steel. This results in carburized steel that has a high-carbon surface and a low-carbon interior. When the carburized steel is heat-treated, the case becomes hardened and the core remains soft and tough. Two methods are used for carburizing steel. One method consists of heating the steel in a furnace containing a

carbon monoxide atmosphere. The other method has the steel placed in a container packed with charcoal or some other carbon-rich material and the heated in a furnace. To cool the parts, you can leave the container in the furnace to cool or remove it and let it air cool. In both cases, the parts become annealed during the slow cooling. The depth of the carbon penetration depends on the length of the soaking period. With today’s methods, carburizing is almost exclusively done by gas atmospheres.

Sealed Quench Furnace work with controlled atmosphere and thermo-chemical treatment of charge take place within the range of temperatures 650-1000°C. The important feature of this furnace is the provision of automatic drive system which transfer the charge in and out of the furnace practically minimizing manual handling and operation fatigue. All phases of heat treatment include the quenching or cooling process is carried out in controlled atmosphere, which prevent any oxidation or decarburizing through contact with air. The equipment is well adapted to facilitate the heat treatment of steel of different qualities.

Stages in SQF

Boosting stage- Penetration of carbon in to the surface of the component

Diffusion Stage- Diffusion of penetrated carbon to the specified depth.

Vestibule cooling stage- Annealing process to get uniform structure throughout.

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Hardening Operation- To harden the carburized case by heating and drastic cooling (quenching) to achieve desired metallurgical parameters like surface hardness, core hardness.

2. LITERATURE SURVEY Literature survey was carried out to understand the insight on how specific techniques can be used to improve the productivity and efficiency of the heat-treatment process

C.A. Stickle in his journal Analytical Models for the Gas Carburizing Process explained that rate of carburizing is proportional to the carbon potential of the furnace atmosphere and surface carbon content of the work load. He also explained that in batch furnaces, the processing cycle can be readily changed for each load of parts and changing the processing cycle for a continuous pusher furnace is very time consuming. On the other hand, the process repeatability in batch furnaces not as much than in continuous furnaces [1].

Olga Karabelchtchikov in his dissertation “Fundamentals of Mass Transfer in Gas Carburizing” explained that carburizing proceeds by CO molecule decomposition, the by-products of the carburizing reactions (CO2 and H2O) act as decarburizing agents and the presence of CO2 even in small quantities requires a high CO concentration to balance this decarburizing action. The purpose of the enriching gas is to react with CO2 and H2O, thus reducing their concentration and producing more CO and H2. he also explained the carbon transfer mechanism which starts with carbon transport from the atmosphere to the steel surface, then surface chemical reactions and finally diffusion of the absorbed carbon atoms towards the bulk of the steel down the chemical potential gradient.[2]

Herring H Daniel in his article “Basket, fixtures and grid for vacuum service” explained that Grids, baskets and fixtures for use in vacuum applications are made high-temperature metallic alloys, graphite, ceramic-fiber composite (CFC) materials or combination designs utilizing multiple materials. Proper design and material selection will maximize useful life and allow a true cost/benefit analysis to be performed. He also explained the selection criteria for high temperature application materials which includes design consideration, physical and thermal properties of the material and operation environment and cast properties. [3]

Kelly, James Wilson and Jason in their technical paper “Materials selection considerations for thermal process equipment” explained that during alloy selection criteria for high temperature service, consideration of mechanical properties at temperature, resistance to oxidation or hot corrosion, and availability. They also reported that chromium, nickel and silicon are three major elements which confer resistance to absorption of carbon. Nickel and silicon lower the maximum solubility of carbon and nitrogen[4]

Herring H Daniel in his article “Heat and corrosion resistant materials/composites: a survey of high temperature alloy selection in heat treating” explains about the basic alloying elements and the primary reason for their selection as components of cast alloys and wrought alloys can help the user understand how these alloys effect the variety of

chemistries offered by manufacturers. The general effect of the various alloying elements is summarized below.[5]

Agarwal D.C and Brill U in their technical paper “Combating high temperature corrosion with alloy 602CA in various environments and industries ” explained all high temperature materials and alloys have certain limitations and the optimum choice is often a compromise between various factors such as the mechanical constraints and compatibility at maximum temperature of operation, environmental constraints as imposed by the process conditions o f high temperature, ease of fabricability and repair, cost-effectiveness and availability. [6]

Dinkar Nandwana, Bhupendra, Nikhil Khandelwal, Tushar Bhargava, Kapil Nandwana and Ganesh Jawale in theri journal “Design, Finite Element Analysis and Optimization of Heat Resistant Casting trays used in Heat Treatment Process” explains a methodology to increase the life cycle of Heat Resistant Casting trays while reducing their total weight. Structural analysis is performed using displacement based Finite Element Method. In depth study of stresses and deformations in the existing tray is performed to optimize the structure [7].

Krishnadev M. R, and Jain S.C, in their journal “Improved productivity through failure analysis” explained about the usefulness of failure analysis in improving productivity through selection of better materials and heat treatment that delay or prevent the onset of fracture in precision forging of aerospace component. Also shown examples from failed cold working dies, hot forging dies and broaches.[8]

Ibraham A Rawabdeh examines various kind of waste in job shop environment by exploring their relationships and finally recommend an assessment method which aims in finding the root cause of the different wastes.[9]

3. PROBLEM STATEMENT

Heat treatment process is identified as one of the bottle necks in the piston pin manufacturing process. Almost 50000 parts are outsourced every month to meet the demand. Productivity improvement of heat treatment process will lead to overall productivity improvement

3.1 Aim

To improve the productivity of the case hardening process of piston pin by 20%

To optimize the lot size for each diameter pin in a sealed quench furnace

To improve the fixture design. 3.2 Objective To review the literatures on application of case

hardening process, heat treatment fixture design and material requirements.

To study the existing process of the charge preparation, lot size, cycle time and sequence of the operation of Sealed Quench Furnace.

To identify the area of improvements and possible causes for lesser productivity.

To study the problem in current lot sizing, process, fixture and suggesting future improvements.

SASTECH Journal 40 Volume 11, Issue 2, Sep 2012

To optimize the batch size and document the improved procedures and measure and validate the process.

3.3 Methodology

Literature review for productivity improvement in the manufacturing industries was carried out by referring journals, books, manuals and related documents.

A detailed study about the charge preparation, lot size, cycle time and sequence of the operation of Sealed Quench Furnace was carried out.

Identified improvements in heat treatment fixture design by proper material selection and finite element analysis and make proposed improvements.

Implemented the process and validated the results after the implementation of solutions.

Standardized the new process with a documented SOP. 4. DATA ANALYSIS

4.1 Data Collection Methodology

The data collection is carried out in following ways

4.1.1 Process Flow Mapping

Fig. 1 Process flow

The process flow of the manufacturing of piston pin is shown in figure 1 and carburizing process is identified as one of the bottlenecks in the piston pin manufacturing after doing the gap analysis and analyzing the outsourcing data which is explained in the next part of the chapter.

4.1.2 Gap Analysis

The company has four sealed quench furnaces, three pit hardening furnaces. Information’s like number of shifts, total time available for production, and other observations from the study are discussed below and gap analysis of Sealed Quench furnace is as show in the table 1. The total capacity planning of sealed quench furnace for a month is 2448 hours where as monthly requirement is 2610 hours. So, from the gap analysis it is found out that there is a requirement of extra 162 hours.

Table. 1 Capacity Planning of SQF

4.1.3 Analysis of outsourcing data

Fig. 2 Outsourcing data for carburizing process

Almost averages of 55,600 parts are outsourced for carburizing process.

Fig. 3 Pie chart of high volume parts

4.1.4 Analysis of Charge Preparation and Fixture Data

The part weight, current charge quantity and the cycle time of the selected item are shown as in table 2.

Table. 2 Part details

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Table. 3 Fixture capacity of selected items

The Sealed Quench Furnace is designed to load capacity of 600Kg (with 50kg safety load) and the table 3 clearly shows that the machine is underutilized since the charge weight is coming less than 600kg. The top grid used to hang the components using hanging rods has only 206 holes to hang the parts and hanging rods weighs 300grams. So, only less quantity of pin was able to accommodate in the fixture. Data required were collected by analysing the records and discussions with employees. The rods used for hanging the parts is identified as one of the waste and by eliminating or reducing this waste will result in productivity improvement.

4.2 Analysis of Less Productivity in Heat Treatment Process

The cause and effect diagram is drawn as shown in figure 4 to identify the cause under man, machine, method and fixture categories affecting less productivity in heat treatment.

Fig. 4 CED for lesser productivity in heat-treatment

According to the cause and effect analysis, why-why analysis is done for root causes “Under utilization of machine” and “Un optimised lot size” as shown in table 4 The analysis has concluded that that for better utilisation of machine the fixture design need to be improved to hold more parts by changing the part holding mechanism from rods to wires.

Table. 4 Why-why analysis

4.3 Analysis of case depth on higher side

Fig. 5 Cause and effect diagram for high case depth

The major causes for high case depth are related to man, machine, method and atmosphere.

The fish bone diagram shows that the carburizing time and carbon potential are the major causes for high case depth.

4.4 Analysis of Fixture Failure

The existing top grid develop crack after 28 days of continues usage and so FEA analysis is taken as next step to select the better design. When the material is repeatedly subject to an average stress which is lower than the materials yield stress, fatigue can occur. There are localized regions with localized stresses which exceed the yield stress which occurs due to surface irregularities, part geometry, or microscopic material defects, or even residual stress in the part left from manufacturing. At these regions of localized high stress, a micro crack develops which in turn forms a stress riser in the material, and on each repeated stress cycle, the crack grows. This will continue until the material fails and a viable crack forms. Approximately 275-300 quench cycles (6 Months) – Top Grid.

Fig. 6 Crack in the fixture

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Fig. 7 Cause and effect for fixture crack

The major causes for crack in top grid are related to man, material, design and atmosphere.

The thermal fatigue and material composition is identified as root cause for fixture failure.

4.4.1 Material properties requirement

Hot strength and creep resistance Hot ductility and thermal shock resistance Thermal fatigue resistance and resistance to carbon

pickup Nickel is used in most grades of heat-resistant

alloys and can range from a fraction of a percent to grades where the percentage exceeds 75%. Nickel serves as a strengthener and austenite stabilizer.

Chromium provides the basic oxidation resistance for heat-resistant alloys. It is normally present in the range of 10-30%.

Molybdenum improves high-temperature strength in alloys.

Niobium is a strengthening element and niobium carbides are stable at high temperature and offer improved thermal fatigue resistance.

Table. 5 Current material

5. PROBLEM SOLVING

5.1 Key Issues Identified and Corrective Actions

In order to utilize the machine capacity of 600kg, the lot size was optimized to increase the charge quantity and improving the fixture design.

In order to increase fixture capacity the top grid design was improved to hold more number of parts.

Replace rods with wires in the new fixture to reduce weight of fixture and to hold more number of parts. Currently 14% of the fixture weight is rod weight, which can be replaced by wires. Furthermore, wires can hold more parts then rods.

Four concepts of top grid design were designed and selection was done on the basis of FEA analysis. The stress and displacement is identified in both structural and thermal analysis.

Parts with case depth on higher side of the specification should reduce by reducing the carburizing time and visual management techniques are adopted to improve the effectiveness of solving the above problems.

In order to reduce the chance of errors being made and to provide guidelines for employees to follow, SOP is developed by updating the current requirements.

5.2 Improvement in Fixture

Figure 8 and 9 explains the improvements done on the fixture and top grid.

Rods have been replaced with wires. Charge size also increased. Trials conducted with change in top grid design to

confirm the pitch of holes. Tests conducted for all the selected parts.

Fig. 8 Fixture improvement

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5.3 Improvement in Top Grid Design

Fig. 9 Top grid design improvement

5.4 Process Capability

Fig. 10 Capability analysis for case hardness process

performance report

The figures 10 show the capability analysis for internal and external case depth and also case and core hardness. The above analysis confirms that the process is highly capable since the Cpk value is 2 for all the process.

5.5 Reducing Higher Case Depth to Mean

A part which is having case depth on higher side was selected and made the process capability. Currently the part is having 1.16-1.18mm range of case depth, where the specification is 0.8-1.2mm for carburizing time of 4.5hours. Testing was done with new PLC program with reduced carburizing time of 3.45 hours. The cased depth dimension reduced to 0.96-1.00 mm which is close to the mean and reduced the waste of over processing and also saved 30 minutes of cycle time. The process capability study before and after the improvement is shown in figure 11.

Fig. 11 Process capability analysis

5.6 Material Selection for Fixture

From the literature survey done it was found out that the nickel percentage contributes to thermal fatigue strength. The optimal nickel percentage requirement for the fixture which should sustain continuous thermal load should be a minimum of 60%. By comparing the same with the existing material it was found out that the nickel percentage is 40%. So for correction a new material has to be found out using CES software. By inputting the newly found out nickel percentage HW grade material has been selected.

BEFORE

AFTER

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Fig. 12 Proposed material

5.7 FEA of Top Grid Design Concepts

Fig. 13 FEA flow chart

5.7.1 Design concepts

Four concepts are defined by giving additional support on the plate as shown in figure 14, so that maximum stress can be reduced.

Fig. 14 Design concepts

Table. 6 Design details

Holes for holding Holes Required

Design 1 277 150-250

Design 2 250 150-250

Design 3 280 150-250

Design 4 244 150-250

5.7.2 Steps Involved

Fig. 15 Material properties input

Fig. 16 Meshing

Fig. 17 Fixing the support holes

Fig. 18 Load applied on small holes

SASTECH Journal 45 Volume 11, Issue 2, Sep 2012

Fig. 19 Load and temperature

Fig. 20 Stress analysis

Fig. 21 Displacement analysis

Fig. 22 Strain analysis

Table. 6. 1 FEA results

The calculation of residual stresses and strains using FEA is done by the application of thermal and structural loads, and results have been plot for stress, strain and displacement for all the design. And from the analysis design 4 has minimum displacement and minimum strain and hence it is proposed for future top grid design

6. RESULTS AND DISCUSSIONS

Part name Maruti Diesel

GL 400 GL 520 KUBO

TA

TATA

4DL

Previous charge size

2250 2250 2250 1680 1680

Planned charge size

3000 3000 3000 2520 2250

Avg. Monthly Order

125000 65000 9000 10000 4000

No of charges- Previous

55.6 28.9 4.0 6.0 2.4

No of charges -Current

41.7 21.7 3.0 4.0 1.8

Cycle time/ charge (Hrs)

11 12 11 11 12

Productivity Improvement

33.33% 33.33% 33.33% 50% 33.93

%

Over-all productivity improvement

12.67% 6.33% 1.00% 1.50% 0.34%

Total time saved (Hrs)

132 86.7 11 12.2 5.3

Cost savings (Rs)

540000 325125 45000 53236 21200

Total money saved (Rs)

984561

7. CONCLUSIONS

Productivity has improved by 21.84% in the heat treatment process of piston pins by increasing the number of holes in the top grid. Also rods have been replaced by GI-wires so that rod weight can be compensated parts. This can be horizontally deployed for pins weighing less than 300 grams.

Four concepts for fixture top grid design have done and one design is selected by doing FEA. Selection of material for heat treatment fixture is done and suggested HW grade Nickel alloy for future fixture material. Process Failure Mode Effect Analysis has done to reduce the potential of failures to occur. Total cost savings for a month is Rs 984561. Total time savings for a month is 247.2 Hrs. We conclude that the study has led to significant improvements in process efficiency.

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8. REFERENCES

[1] Stickels C.A, Analytical Models for the Gas

Carburizing Process, Metallurgical Transactions B, Volume 20 B, August 1989, pp 535-546.

[2] Olga Karabelchtchikov, Fundamentals of Mass Transfer in Gas Carburizing, Material Science and Engineering, Vol. 107, August 2007, pp 1 – 135.

[3] Herring H Daniel, Basket, fixtures and grid for vacuum service, International journal of Thermal Technology, Industrial Heating, Bangalore, April 27, 2011

[4] Kelly, James and Wilson, Jason, Materials selection considerations for thermal process equipment, Heat processing (3), Issue 3, 2005.

[5] Herring H Daniel, Heat & corrosion resistant materials/composites: a survey of high temperature alloy selection in heat treating, International journal of Thermal Technology, Industrial Heating, Bangalore, Sept1, 2006.

[6] Agarwal D.C, Brill U, Combating high temperature corrosion with alloy 602CA in various environments and industries, Krupp VDM Technologies, paper no-02372, Corrosion 2002.

[7] Dinkar Nandwana, Bhupendra, Nikhil Khandelwal, Tushar Bhargava, Kapil Nandwana and Ganesh Jawale, Design, Finite Element Analysis and Optimization of Heat Resistant Casting trays used in Heat Treatment Process, Proceedings of the World Congress on Engineering 2010 Vol 2, Bangalore, 2010, London, July 2010.

[8] Krishnadev M. R, Jain S.C, Improved productivity through failure analysis, Engineering Failure Analysis 14 (2007) pp-1053–1064.

[9] Ibrahim A. Rawabdeh, A model for assessment of waste in job shop environment, International journal for operation and production management, Vol 25 No.8, 2005 pp800-822.