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95

WHAT IS THE OPTIMUM BASE SULPHURCONTENT PRIOR TO NODULATISATION?

byReginald D. Forrest, Sorelmetal Technical Services

Sulphur control in the production of Ductile Iron

has long been recognized as being essential.Most foundries are fully aware of the dangersinvolved in allowing the level of sulphur toincrease in the base iron. The risk is under-treatment (Mg content too low) with consequentpoor graphite shape (Figure A). Higher sulphurlevels call for increased additions of nodularizingalloy with consequent higher treatment costs anddangers of sulphite dross defects (Figure B).With these higher sulphur levels there is anincreased tendency for nodularity fade and ofcourse problems in meeting requiredspecifications.

FIGURE A. Under-Treatment with Poor Nodularity.

What is not so well recognized are the dangersof too low a sulphur content. When thesulphur content in the base iron is very low - forexample 0.004% S and a magnesium treatmentis made which results in a residual Mg of say0.045% Mg, then the effective Mg level is toohigh. The result is that undesirable carbides(chill Figure C) may be formed with all of theunwanted problems of high hardness, lownodule counts, poor graphite shape, increased

shrinkage, and poor machinability and

brittleness. These problems with excessive Mgdo not take into account the additive effect of Ceas a desulphurizer and nodularizer.

FIGURE B. The Microscopic Appearance of Magnesium Silicate Inclusions.

Indeed, there is increasing evidence that smallamounts of sulphides (possibly Ca, Ba, Ceand/or Mg) may assist the nucleation of graphiteas well as influencing the surface tension of theliquid phase and thereby further influencing thenucleation and growth of graphite, Figure D.

RIT Technical Services metallurgists, as well as

many others in the field, have been frequentlyconfronted with problems associated withunsatisfactory graphite forms, low nodule counts,carbides etc. in Ductile Iron. There can be manyand varied causes for such structures - includingthe presence of subversive elements,metal/mould reactions, excessive amounts ofcarbide stabilizing elements, etc. - but unsuitableand variable levels of sulphur in the base ironhave, in our experience, often been found to beresponsible for many of these such problems.

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FIGURE C. Carbides (chill)

FIGURE D. Optimum nodular graphite shape

In order to control and avoid the problemsassociated with sulphur in Ductile Iron thereforeit is essential to regularly measure (analyze) thesulphur content of the liquid base iron prior tonodularizing. In foundries where inductionmelting is used, this is a fairly easy control toestablish and monitor frequently. In cupolamelting foundries, where desulphurisation priorto nodularizing is practiced, it becomes a littlemore problematic since the effectiveness of thedesulphurisation can vary depending on severalfactors. Nevertheless, it is necessary to knowwith reasonable accuracy the sulphur content of

the iron prior to the Mg treatment. It isRIT/Sorelmetal experience that the optimumlevel of sulphur in the liquid base metal, prior toMg treatment should be approximately 0.01% S(+/- 0.002). Above and below these levels thereis increasing risk of encountering problems.

WHAT MUST BE DONERegularly and routinely check the sulphurcontent of your base liquid iron prior tonodularization.

Where the sulphur level of the base iron istoo high, action is required to reduce it.

Use charge materials of known and consistentanalysis (Sorelmetal), low S graphite.

Desulphurize metal. Change melting practice.

Where the sulphur level of the base iron istoo low (below 0.008%) it is necessary toincrease it in order to bring it within therecommended range (0.008 - 0.012% S).

Where melting is made by Induction or rotaryfurnaces, this can be done easily by thecontrolled addition of iron sulphide (iron pyrite).For each 0.001% of sulphur increase required anaddition of 30 g (0.067 lb) FeS per ton should bemade.

(eg. Where the base S content of a 1 ton ofliquid base iron is analyzed at 0.004% S anaddition of 180 g of pyrite is required).

Where cupola melting followed bydesulphurisation (eg. porous plug / calciumcarbide) is practiced, it is necessary to

constantly adjust the practice (amount of carbideaddition or dwell time based upon thetemperature of the metal and degree ofagitation).

In situations where pure magnesium is used as aone step desulphurisation/nodularization treat-ment, the control of the sulphur content is lesscritical because the reaction removes most ofthe sulphides. However, in this case theinoculation practice becomes more difficult andother materials need to be considered to achievegood nodularity.

REFERENCES

1. The effect of sulphur on the occurrence ofcarbides in nodular cast iron. Paper by P.C. vanEldijk presented at the 53

rdWorld Foundry

Congress, September 1986.

2. Ductile Iron Handbook, published by AFS -Chapter 4 pages 77-78.

3. Numerous unpublished QIT/RIT investigationalreports and field experiences.

4. Also see Suggestion Sheet 76.

REV March 2006