most used treatment alloy for a nodulizing treatment [1.0 - 3.0 wt-% addition], and/or in FeSi-based inoculation alloys [0.10 - 13.0 wt-% REE content], used at 0.10 - 1.0 wt-% addition for Mg-treated iron.23-27
At the present time, many
foundries are looking at reducing the REE levels in magne- sium ferrosilicon, as the result of excessive price volatility of the REE sources.28, 29
Control of Anti-Nodularising Elements
An anti-nodularising effect is induced by Al, As, Sn, Ti, Sb, Pb, Bi, whereas intercellular flake graphite can be promoted by Al, Ti, Sn, Bi, Pb, Sb, As. Chunky graphite is linked to the presence of Si, Ni, Ce, Ca or exploded graphite (Ce). Reduced machinability is caused by carbides from these Mn, Ti, V, Cr, Mo, B, H, Se, Te.21, 22, 30 - 32
Generally, in Mg treated structure.30 - 33 Px
irons, the complex Thielman’s factors K (anti-nodularising action) and Px
(pearlite formation) can characterize the final :
= 3.0 (%Mn) – 2.65 (Si - 2.0) + 7.75 (%Cu) + 90 (%Sn) + 357 (%Pb) + 333(%Bi) + 20.1 (%As) + 9.60 (%Cr) + 71.7 (%Sb)
K = 4.4 (%Ti) + 2.0 (%As) + 2.4 (%Sn) + 5.0 (%Sb) + 290 (%Pb) + 370 (%Bi) + 1.6 (%Al)
Eqn. 1 Eqn. 2
The anti-nodularising influence of elements must be con- sidered in ensuring acceptable levels of graphite nodularity, defined for ductile iron as > 80% nodular (spheroidal) graph- ite (NG) and < 20% vermicular (compacted) graphite (VG), with no lamellar graphite (LG). Compositional control be- comes critical when higher values for graphite nodularity are specified (up to 100% NG), such as for windmills castings, and especially when maximum compactness for NG must be achieved (e.g. for type K-ASTM irons).33
Excessive Ce, in the absence of tramp elements that promote flake graphite, especially in heavy section regular ductile iron, will almost always cause chunky graphite and explod- ed graphite. At high cooling rates, as in thin wall castings, excessive residual REE contents are one of the most impor- tant causes of carbide formation.
In a previous experimental program,34 as-cast and heat
to medium and high levels (up to 0.4%Mn, up to 0.045%P, Px
> 3.0). A summary of results is presented in Table 1.34
The influence of manganese was found to depend on the phos- phorus and residual elements level. The base conditions to ob- tain an as cast ferritic structure, as required by the 400–18 ductile iron grade specification, was < 0.03%P, ≤ 0.2%Mn and Px
of pearlite in the as cast structure, although a ferritic struc- ture is obtained after a short annealing heat treatment. Main- taining low levels of phosphorus (< 0.025%) and residual elements (Px
<2.0).
treated 400–18 grade ductile irons were prepared in core- less induction furnaces under different foundry conditions using various metallic charge compositions and qualities: 0% - 30% molten iron ‘heel’, as grey iron or ductile iron; 26% - 40% ferritic ductile iron returns; 18% - 40% steel scrap at different qualities; 0 - 47% high purity pig iron. Mg treatments recorded with and without REE present: 1.7 - 2.2%FeSiCaMg or FeSiCaMgRE as nodulisers. Inocula- tion parameters: 0.5% - 0.8% FeSi75 or Ca,BaFeSi alloys, as ladle inoculation. The final melts ranged from low Mn, P and residual element contents (Mn < 0.2%, P < 0.03%, Px
< 2.0)
< 2.0. At the same low levels of Mn and P, increasing the residual element content (Px
> 2.0) leads to the presence
< 2.0) allows relative high manganese contents (0.32%–0.38%) while maintaining a ferritic structure in the as cast state. High phosphorus (0.04% - 0.045%) and manganese (0.25% - 0.35%) contents led to a stabilized pearlite micro- structure, even at low residual element contents (Px
Table 1. Summary of Mechanical Property Test Results
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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