Economic aspects of welding process.

Popescu, Mihaela ; Vartolomei, Mihaela ; Mocuta, Georgeta 等

1. INTRODUCTION

Under nowadays context, practical field companies' preoccupations are really looking for efficiency of their activities (to find solutions to reduce costs for welding activities and to increase the productivity), optimization of consumptions (solutions to find the minimum costs of products but respecting required quality standards), homologation and launching new products, with respect of the great competition conditions on the market (Izvercian, 2002). The audit of the whole welding activity is compulsory in order to identify types of the adequate consumables and welding equipment (Rosenthal, 2001).

The objective of the firm (in the neoclassical model) is to maximize profits. Firms are unable to affect the prices of either their inputs or their outputs (Vartolomei, 2009). The optimal consumption bundle is the answer to consumer's maximization problem. An optimal consumption bundle is the affordable bundle of goods that makes the consumer the happiest.

The following properties may or may not hold for a particular production set: irreversibility, possibility of inaction, production with a single output, cost minimization. Usually, if the production set has one of these properties, it will be easier to choose the profit-maximizing bundle.

Consider the model of profit maximization when there is a single output. At price p, let z(w, p) be the firm's factor demand function and the firm's supply function is

q (w, p) = f(z (w, p)) (1)

At a particular price vector, (w*, p*), let be

q* = q (w*, p*) (2)

An interesting implication of the profit maximization problem is that

z* = z (w*, p*) (3)

solves the following problem:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

s.t.:f(z) [greater than or equal to] q * (5)

That is, z* is the input bundle that produces q* at minimum cost when prices are (w*, p*). If a firm maximizes profit by producing q* using z*, then z* is also the input bundle that produces q* at minimum cost. The fact that cost minimization is necessary for profit maximization points toward another way to attack the firm's problem. First, for any level of output, q, find the input bundle that minimizes the cost of producing that level of output at the current level of prices. That is, solve the following problem known as the Cost Minimization Problem:

min w x z (6)

s.t.:f(z) [greater than or equal to] q (7)

2. MOTIVATION

Sometimes punctual actions are necessary which may suppose both diagnoses of the welding activity actual state, and settle the objectives and the solutions to reach these required objectives.

The effective implementation of the solutions also involves their control on a certain determined time period.

Though companies may not realize it, whenever a welder has to stop his work to search for a part on the ground around him, even if the part is within easy reach, time and energy is wasted. Closed related to this perpetrator of wasted effort is the bad habit of handling parts more often than necessary. An evaluation method that helps this task is very important to the final results of any Company market strategy. A comprehensive and precise analysis to select correctly a welding process in real situations is very hard and complex, because of many variables involved. An important aspect is that, in the market strategy, quality and costs, as the other requirement, need to be analyzed as a whole. At first glance, welding costs seem to have a more measurable property. However, they involve a great number of components, such as welding execution, process selection, staff training, joint design, equipment definition/setting and even fabrication simulation. The determination of welding costs requires considering welding parameters and prices of consumables, workers, equipment, etc. One must give close attention to the relevant components of costs during the determination and control/reduction of them. Similarly to quality, a target for low costs depends on the particular application. An interesting way of gauging inefficiency in parts handling is to mark a piece with soapstone every time the piece is picked up. Using this method, managers are often surprised to see how many times a part is actually handled for no reason.

Technology is now available that can circumvent the headaches involved in record keeping. Many current software packages can be tailor fit to the specifics of the individual company's needs and provide a great amount of efficiency while eliminating the margin for human error. Another way to use technology in reducing costs is to introduce robotics or automation to the process. Robotics is only necessary in cases where the volume of parts to be welded becomes so great that cost of human labor exceeds cost of setting a robotic solution.

[FIGURE 1 OMITTED]

Automation can be ideal for smaller operations that simply want boost quality and efficiency (Figure 1).

3. RULES FOR WELDING ECONOMICS

Companies wishing to reduce their welding costs can benefit from the following rule of thumb: labor and overhead account for 80% of the total costs associated with mild steel welding operations, while the cost of materials accounts for only 20% (Escala et al, 2006). In other words, companies wishing to save money on welding would be better off concentrating their efforts on trimming the procedural aspect of their operations. What follows is a series of steps that companies can take to realize savings in these areas (Hachicha, 2007).

There are other areas of inefficiency that companies can look out for as well. These include examining such variables as weld diameter, wire feed speed, voltage, travel speed, gas type and transfer mode, to name just a few of them. Another area to be aware of is joint preparation. Welders should use a double-bevel whenever it is possible, rather than a single-bevel. This simple adjustment in procedure can save a great deal in terms of unused weld metal (Rosenthal, 2001).

Companies would also do well to change the design of a welded part to eliminate the need for unnecessary welds. For obvious reasons, extra weld waste time, effort and materials. Companies could also ultimately reduce production costs by keeping their eyes open for items that could be welded rather than cast. There are many cases where it may be more cost effective to a part than to cast an entire component in a costly alloy or exotic metal (Silva et al, 2000).

The authors proposed the following rules to be considered:

* Adopted simple details for welded attachments.

* Avoided excessive weld volume, i.e. do not overspecify weld size.

* Used fillet welds in preference to groove welds, provided the size is not too large.

* Remembered that longer, smaller fillet welds are cheaper than shorter ones of larger size.

* Used partial penetration welds, in prepared butt joints, in preference to full penetration welds when the force on the joint will allow.

* Avoided expensive plate edge preparations for groove except where the fabricator is equipped for such preparations.

* Arranged for as much welding to be done in the shop and as little as possible at site.

* Used the down hand welding position.

* Avoided having to turn members or assemblies over to welds.

* Ensured adequate accessibility of the electrode to all welds.

* As a designer does not specify in too great detail the manner of making a weld; the fabricator is in a better position to decide on the most economical joint preparation and welding procedure.

* Used bent or folded plates for corner joints in preference to welding.

* Used prequalified joints and welding procedures to avoid qualification testing.

* Does not call for unduly stringent inspection.

First, companies need to pay careful attention to the delivery of both consumables and accessories to the welding site. Operating efficiency can be improved simply by stocking sufficient supplies of all items necessary to the welding process within an area immediately accessible to the welder. Having these items, which can include shielding gas, flux and wire, within easy reach reduces the time the welder needs to move from the welding station and therefore saves on labor costs.

Incidentally, it is also a good idea to switch to larger spools of wire to reduce the frequency of changeover times, which can add up in cost as well.

Another way to reduce costs is to guard against overwelding (Silva et al, 2000). Oftentimes, welders put in a slightly larger weld than is sufficient. Whether they are doing this out of ignorance as to the proper length of the weld cut or because they are trying to ensure that enough weld metal is in place, the end result is that valuable consumables are wasted Not only is the company paying for wasted consumable material, but the overwelded material is more likely to end up warped or distorted due to the extra heat that was applied in the welding process.

4. CONCLUSION

This paper has studied the welding process through the perspective of manufacturing expenses because costs optimization for welding and allied processes (thermal spraying, cutting, brazing and soldering etc) represents the preoccupation of all companies. So, it supposes a profound, careful analysis of all activities and preparatory-intermediary stages of welding and control, respectively (Hachicha et al., 2007) this paper has emphasized. In conclusion, it is imposed to reach clear targets: cost calculation, audit and analyses under all aspects of the used welding and cutting processes, the performance and productivity of welding and allied processes.

5. REFERENCES

Escala, S.; Nooij, M. & Quintino, L. (2006). Economically welding in a healthy way, Proceedings of The 7 South-East European Welding Congress, ISIM Timisoara, IIW/IIS, Romania, pg. 244-251, ISSN 8359-40-6, Timisoara, May 2006, Sudura Publishing, Timisoara

Hachicha, W.; Masmoudi, F. & Bouaziz, Z. (2007). A new feature-concept applied in cost estimation model for a weld assemblage, Advantages in Production Engineering and Management Journal, Vol.42, No 2, (January, 2007) pg. 149-162, ISSN 1854-6250

Izvercian, M. (2002). Marketing Elements; Solness Publisher, ISBN 973-8472-19-9, Timisoara

Rosenthal, J.W. (2001). Ten steps to reducing your welding costs, Welding Journal, vol.79, (July 2001) pg. 32, ISSN 0043-2296

Silva, C.R.; Ferrares, V.A. & Scotti, A. (2000). A quality and cost approach for welding process selection, Journal of the Brazilian Society of Mechanical Science, vol.22, no 3, (March, 2000), ISSN 0100-7386

Vartolomei-M, M. (2009). Contribution to the management of change and energy in military industry in the frame of suprastate play, Politehnica Publisher, ISBN 978-973-625-803-9, Timisoara