Problem Solving Methodology for Preventing Lump Defects in Ductile Iron Pipe production Amitava Sengupta TATA Metaliks DI Pipes Ltd, Kharagpur, West Bengal, India Email :
[email protected] Mob.: +91 9433450921
ABSTRACT Slag buildup in the Pouring ladle has been a continuous challenge. Buildup also occurred in the Mg-treated metal in transfer ladles. However, the major problem area was significant buildup downstream fixed pouring ladle at CCM area, huge slag sticking, erosion, less lining life of the fixed ladle, insoluble build-up in transfer ladles and treatment ladles has caused problems increasing defect due to” lump” of Ductile Iron Pipe (DIP) manufacturing which one of the cause of pipe leakage & lump defect rejection. By Understanding the organizational policy “…. Practice of continual improvement …” an improvement initiative has been taken. In this paper adopted the problem solving methodology by establishing a new practice by changing the framework of current practice is discussed. Improvement in quality of pipe by modification of ladle design and applying customized special type of improvised refractory material and flux to reduces dross entrapment during casting generated internal customer satisfaction (finishing line). This minimized rework and controlled rejection. KEYWORDS: Plan-Do-Check-Act (PDCA), Critical to Quality (CTQ), Define-Measure-Analyze-Improve-Control (DMAIC), Supplier-Input-Process-Output-Customer (SIPOC), Voice of Customer (VOC),Voice of Business (VOB), Work In Progress (WIP), Induction Furnace (IF), Liquid Metal (LM), Mg treatment (Mgt), Centrifugal Casting Machine (CCM).
PROBLEM STATEMENT
TASK ACHIEVING STEPS
Huge slag sticking, erosion, less lining life of the fixed ladle, insoluble build-up in transfer ladles and Mg treatment ladles has caused problems, increasing defects due to “lump” of DIP manufacturing Process.
Table 1 shows the activities for the Task achieving.
CONCEPT OF TASK ACHIEVING The activities to achieve the intended target by establishing a new practice or by breaking through the framework of current practice, the focus of problem solving is when process has already been established and trying to tune the existing system for quality, cost or productivity improvement, task-achieving focuses at the design and development stage of process while constructing a new system.
Table 1: stepwise activity Steps
Activity
1
Define the task to be achieved and its scope, select team members and make the time schedule for the project
2
Understand and capture internal customer requirements (VOC), Voice Business requirements (VOB) for developing the newer material and translating it to technical requirements.
3
Gather internal data for rejection due to “Lump”
4
Explore various possible design alternatives to develop modified ladle, then evaluation of design based on cost and selection the best
5
Identify CTQ, Detailed designing , Study SIPOC Process Map of Current Practice and develop Process Flow of new practice
6
Evaluation of product at internal customer’s end (CCM casting area, and ladle refractory management department)
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PROCEDURE FOR TASK ACHIEVING
IDENTIFY THE CTQ
Table 2 shows the detailed procedure for carrying out the task achieving.
Table 3: CTQ product/service characteristic that satisfy Customer or Process
Table 2: Task achieving phase PLAN
1. Understanding the organizational policy “… Practice of continual improvement …”
VOB Less Dross Entrapped Pipe due to lump(Internal Customer)
% Rejection due to lump
% Rejection due to lump
Process time, Cost Efficiency, Rework
Critical to Cost
8. Standard Operating Procedure update
VOC We are not able to process casted pipe with in the time due to lump
9. Future Plan (All pouring casting Ladle shall be made with modified T Pot ladle with SiC ladle liners
PROBLEM SOLVING PHASE
4. Explore a successful scenario in the trial application 5. Implement the Scenario
CHECK 6. Confirm the effects 7. Transfer to the daily Operation Casting Department
VOCQuick delivery of Pipe
Important Measure Critical to Delivery
3. Develop the methods to perform the task ( New drawing development of Sic ladle liners)
ACT
CTQ Time
2. Set up the task (Newly design modify ladle and use of special type of SiC based refractories & Sourcing of special type of Flux as Mg dross arrestor)
DO
VOC/VOB
(External Customer)
Table 4 Problem Solving Phase wise activity SIPOC PROCESS MAP CURRENT PRACTICE
Phase Define
Description Huge slag stick in the ladle—Dross formation Huge refractrory lining errosion—less liling life
Measure
Visual assessment of slag build up in the ladle Defect data collected & analysis of current prctice
Analyze
Trend analysis, Process Capability(Cp,CpK)
Improve
Redesigned Tea-pot ladle with SiC ladle liner Application of Special type of Fluoridefree Ladle Flux
Fig. 1 Process Flow before improvement
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Control
SOP made for Process Improvement
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PROCESS FLOW NEW PRACTICE Figure 2 shows the process follow diagram for the DIP manufacturing after the improvements.
Fig.4 (Before improvement) Insoluble build-up in transfer ladle
WHAT ARE DROSS DEFECTS The addition of Mg to liquid cast iron is an essential processing step for manufacturing ductile iron. Magnesium, however, is a very reactive element. As a consequence, ductile iron has a higher tendency to form slag. The reaction products that result are often called dross. These defects in pipe castings associated with Dross-Slag rich in Mg & S –formed as a result of oxidation and desulphurization reactions during and after the Mg treatment Process. Dross defects may be one of the causes of Pipe Leakage, lump defect. DROSS DEFECTS IN DI PIPE CASTING Fig. 3 To Fig 7 shows the various dross defects before treatment and after treatment and also news designed the ladle for the ductile iron pipe manufacturing.
Fig.3 Magnesium dross stringers
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Fig.5 (After Improvement) Insoluble build-up reduced
Fig.6 Newly Design Modify Ladle and Use of Special Type of Sic Based Refractories
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% of reec tionj due to lump with respec t to total rejection
Trend analysis after inprovement 14
13
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10
9
8 1
2
3
4
5
6
7
8
9
Month ( July to March)
Fig.10 Trend analysis after improvement
Fig7 Pre cast Pre-fired shape of SiC Impact pad (Ladle Liners) outstanding hot strength, resistant to thermal shock, excellent resistance to FeO & other corrosive oxides
Process Capability Report a fter improvement LSL
USL
Process Data L SL 8.59 USL 13.31 Sample Mean 10.27 StDev(Overall) 1.55995
Overall Capability Potential (Wi thin) Capability Cp 0.63 Cpk 0.45
RESULTS OF IMPLEMENTATION
% of rejection due tolumpwith respect tototal rejection
Fig.8 to Fig 11 shows the trend analysis of the before and after improvements in Ductile iron manufacturing process and also process Capability. Trend analysis before improvement 21
20
7
19
9
10
11
12
13
14
Fig.11 Process capability
18
17
PROCESS CAPABILITY
16
15 1
2
3
Month ( April to June)
Fig.8 Trend analysis before improvement % of rejection due to lump with respect to total rejection
8
O ve rall trend ana lysis
Cp value indicates an improvement in the process, if Cp =1 i.e. Process is just Capable so there is an OFI and CpK = 1 i.e. the process is expected to produce at least 99.7% good so there is still an Opportunity for continual improvement and further study is require.
22. 5
20. 0
STANDARDIZATION
17. 5
I.
Preparation of Standard Operating Procedure (SOP)
ii.
Necessary On Job Training (OJT) imparted.
iii.
Define mechanism (system, assign process owner & responsibility) to monitor/audit on check the SOP are being followed.
15. 0
12. 5
10. 0
1
2
3
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6
7
8
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12
Month ( April to March)
Fig.9 Overall trend analysis
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LEARNINGS
SUMMARY
DEFECTS
Breaking thru, Area
i.
Manufacturing Defects
ii.
Quality Defects
iii.
Occurrence of Defects a.
Manufacturing defects mean cast DI Pipe have to be reworked or repaired
b.
The total manufacturing cost of DI Pipe includes the cost of producing defects.
c.
DI Pipe customers are not willing to pay for the cost of making defects.
d.
Making defective DI Pipe is costly for DI Pipe manufacturing plant.
e.
Defects interrupt the smooth flow of DI Pipe production line and generates WIP
f.
The DI Pipe manufacturing process must check and correction shall be taken when Operators cannot prevent defects.
Redesigned Tea-pot ladle with SiC ladle liner
Modified Tea Pot Ladle for Centrifugal Casting Machine Shop are used for the production of Ductile iron pipe casting. In the ladle itself, there is a SIC ladle liners which placed in the bottom of the ladle to arrest Insoluble build-up in transfer ladles. The advantage of this improvement activity to achieve the intended target by establishing a new practice and breaking thru the framework of current practice.
Application of Special type of Fluoride-free Ladle Flux
Insoluble build-up in transfer ladles and treatment ladles has caused problems increasing defect due to” Lump” this fluxes provide a safer alternative for effective ladle cleaning. By applying these fluxes in ladles, and using proper de-slagging techniques, we have gained a renewed confidence in the use of fluxes.
IMPACT i.
Tangible: Financial Savings Intangible: Meeting Customer Expectation, KPI Improvement, Benchmark
g.
Operators, first priority is to pay attention to Quality
ii.
h.
Operators shall check the Quality of what they have made at their own workstation.
CONCLUSIONS i.
Sum up remaining problems
ii.
Plan what is to be done to solve these problems
iii.
Documented learning’s form the improvement initiative (what went wrong and what should be replicated etc.)
iv.
Look for horizontal deployment opportunity
IMPROVEMENT INITIATIVE i.
Improvement initiative activities identify problems & develop solutions
ii.
Improvement initiative team members shall discuss & analyse problems & consider solutions
iii.
Improvement initiative activities improve working conditions & also provide opportunities for self-development.
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ACKNOWLEDGEMENTS The Author wishes to express his profound gratitude to the management of Tata Metaliks DI Pipes Ltd for giving opportunity for the improvement initiative - and also to fellow colleagues Department of Centrifugal Casting Machine, Quality Assurance for giving Co-operation for the initiative success. REFERENCE Sofia Andersson, Study of Dross in Ductile Cast Iron Main Shafts.
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