In the development of marine resources, there is a significant demand for corrosion-resistant materials. Nodular cast iron, due to its low cost, excellent wear resistance, and good castability, holds broad application prospects in marine engineering. However, its corrosion resistance is often limited, restricting its use in certain areas. STQNiCr is a nickel-chromium alloyed nodular cast iron primarily used for manufacturing valves and other industrial equipment resistant to seawater corrosion. The addition of nickel promotes the formation of a more stable and dense passivation layer on the material surface, better blocking erosion from various corrosive media. This allows castings to maintain stable performance over extended periods in different corrosive environments, such as acidic or alkaline media, compared to samples with lower nickel content. Silicon not only increases ferrite content but also enhances the corrosion resistance of nodular cast iron, offering excellent tensile strength, elongation, and a high yield ratio. Therefore, high-silicon solid solution STQNiCr nodular cast iron has even broader application potential. In this study, I investigate the influence of nickel content on the microstructure and mechanical properties of this advanced nodular cast iron.
The nodular cast iron industry continuously seeks to optimize alloy compositions for improved performance. The role of nickel in ferrous alloys, particularly in nodular cast iron, is multifaceted, affecting phase stability, mechanical strength, and corrosion behavior. This research focuses on a specific high-silicon variant of STQNiCr nodular cast iron, where silicon is maintained at elevated levels to promote ferrite formation and enhance corrosion resistance through solid solution strengthening. The interplay between nickel, chromium, and silicon in the matrix of nodular cast iron is complex and warrants detailed examination. Understanding how nickel content modulates the microstructure—specifically the pearlite-to-ferrite ratio and the potential formation of undesirable phases like carbides or ledeburite—is crucial for tailoring properties to meet demanding specifications for marine applications.
The selection of raw materials is fundamental to achieving the desired chemical composition and, consequently, the properties of nodular cast iron. Based on extensive production experience with nodular cast iron, the charge composition was carefully designed to meet mechanical performance requirements while minimizing the use of returns. The charge ratio is summarized in Table 1.
| Charge Component | Proportion (wt.%) |
|---|---|
| Pig Iron | 60 |
| Steel Scrap | 15 |
| Returns | 25 |
Melting was conducted to produce Y-block samples with a main wall thickness of 40 mm and a molten metal weight of 500 kg. The target specifications required a tensile strength (Rm) ≥ 500 MPa, elongation (A) ≥ 3%, carbides ≤ 1%, chromium content ≥ 0.65 wt.%, and nickel content ≥ 1.0 wt.%, with higher nickel content being preferable for enhanced corrosion resistance, provided mechanical properties are satisfied.
Chemical Composition Design and Control
The chemical composition is the cornerstone of controlling the microstructure and properties of nodular cast iron. The elements carbon, silicon, chromium, and nickel play particularly critical roles in high-silicon STQNiCr nodular cast iron.
Carbon and Silicon
Carbon content was maintained between 3.45 wt.% and 3.55 wt.% to ensure good fluidity and graphite formation. Silicon is a potent ferrite stabilizer. Silicon atoms dissolve into the iron lattice, altering the crystal structure and electron cloud distribution, which reduces the stability of austenite (face-centered cubic structure) and promotes ferrite formation. Furthermore, silicon decreases the solubility of carbon in iron, encouraging carbon to precipitate as graphite rather than forming carbides like cementite. This graphitizing effect is crucial for achieving a fully nodular graphite structure. From a corrosion perspective, silicon enables the formation of a protective SiO₂-based oxide film on the casting surface via electrochemical reactions. This film effectively isolates the iron matrix from corrosive media, slowing the corrosion rate. Silicon also increases the electrode potential of nodular cast iron, reducing the proportion of iron acting as an anode during electrochemical corrosion. As silicon content increases, the electrode potential gradually rises, enhancing corrosion resistance. Therefore, controlling silicon content is key to optimizing both ferrite content and corrosion performance. In this study, silicon was set within the range of 3.30 wt.% to 3.50 wt.%.
The combined effect of carbon and silicon can be approximated by the carbon equivalent (CE) formula, commonly used for cast irons:
$$ CE = \%C + \frac{\%Si + \%P}{3} $$
For the compositions used, the CE ranges approximately from 4.4 to 4.6, indicating a hypereutectic tendency which favors graphite precipitation during solidification.
Chromium
Chromium, at low levels, hinders ferrite formation and stabilizes cementite within pearlite. After heat treatment, this can lead to a spheroidized pearlitic structure. However, as chromium content increases, the eutectic point shifts to the right, reducing the temperature gap between the stable (graphitic) and metastable (carbidic) eutectic reactions. This increases the tendency for carbide formation and can lead to chill (white iron) structures. To avoid excessive hard phases and maintain ductility, chromium content was set at the minimum required value of 0.65 wt.%.
The influence of chromium on carbide formation can be described by a stability parameter. The tendency for carbide formation instead of graphite increases with the following ratio:
$$ \text{Carbide Tendency} \propto \frac{\%Cr}{\%Si} $$
Maintaining a low ratio is essential to prevent chill in nodular cast iron.
Nickel
Nickel is a strong austenite-stabilizing element. Its effect on the pearlite and ferrite balance varies with content, generally promoting the formation of an austenitic matrix upon solidification, which subsequently transforms. At low nickel content (around 1 wt.%), a mixed pearlite-ferrite matrix is typical. As nickel content rises, the pearlite content increases because nickel lowers the eutectoid transformation temperature and slows the diffusion of carbon, favoring a finer pearlitic structure. Nickel has minimal direct impact on graphite nodule size and distribution but can indirectly reduce graphite distortion by stabilizing the matrix, helping maintain spherical graphite integrity. Importantly, nickel suppresses the precipitation of carbides at grain boundaries. Through solid solution strengthening and austenite stabilization, nickel enhances tensile and yield strength. For corrosion resistance, nickel promotes the formation of a more protective passivation film (e.g., mixed chromium/nickel oxides), improving performance in acidic, alkaline, and chloride-containing environments. In seawater or industrial atmospheres, the corrosion rate of nodular cast iron significantly decreases when nickel content exceeds approximately 2 wt.%. To systematically study its effect, three different nickel levels were chosen: 1.0 wt.%, 1.5 wt.%, and 2.0 wt.%. The complete target chemical compositions are listed in Table 2.
| Sample ID | C | Si | Mn | P | S | Cr | Ni |
|---|---|---|---|---|---|---|---|
| C1 | 3.45-3.55 | 3.30-3.50 | <0.2 | <0.06 | <0.015 | 0.65 | 1.0 |
| C2 | 3.45-3.55 | 3.30-3.50 | <0.2 | <0.06 | <0.015 | 0.65 | 1.5 |
| C3 | 3.45-3.55 | 3.30-3.50 | <0.2 | <0.06 | <0.015 | 0.65 | 2.0 |
Nodularizing and Inoculation Treatment
To achieve the characteristic spherical graphite morphology in nodular cast iron, a two-step treatment process is essential: nodularizing (or spheroidizing) and inoculation.
Nodularizing Treatment
Nodularizing treatment converts graphite from flake to a more regular spherical shape. This dramatically reduces the notch effect and stress concentration within the matrix. Under external force, the material can undergo plastic deformation more uniformly, making it less prone to crack initiation and propagation at graphite sites, thereby improving elongation. For instance, nodular cast iron with high nodularity exhibits a substantial increase in elongation compared to ordinary gray iron. The treatment also promotes uniform distribution of graphite nodules, avoiding local performance variations due to graphite clustering or poor morphology, leading to more consistent and reliable elongation across the material. Typically, nodularizing increases ferrite content. However, in this alloy, the addition of chromium and nickel significantly increases pearlite content and may even promote carbide formation, which can reduce elongation. Therefore, the amount of nodularizing agent used was higher than for standard nodular cast iron grades, specifically 1.6 wt.% of the treated iron.
The efficiency of nodularization can be related to the residual magnesium content. A common aim is to maintain a residual Mg level above 0.03-0.05 wt.% to ensure good nodularity. The reaction can be simplified as:
$$ \text{Mg (added)} \rightarrow \text{Mg}_{res} + \text{MgS} + \text{MgO} $$
Sufficient Mg must remain to alter the graphite growth kinetics.
Inoculation Treatment
Inoculation provides numerous nuclei for graphite precipitation, ensuring a more complete graphitization process and preventing the formation of cementite during solidification. It helps guarantee that graphite precipitates in a spherical form, with fine, round, and uniformly distributed nodules, which minimizes stress concentration. Consequently, the strength, toughness, and wear resistance of nodular cast iron are significantly improved. Inoculation also effectively mitigates microstructure inhomogeneity caused by segregation, increasing material density. By reducing intergranular segregation, it lowers the corrosion potential difference due to compositional variations, thereby enhancing the corrosion resistance of nodular cast iron in various environments and extending component service life. In this experiment, the total inoculation amount was 1.25 wt.%. Since silicon-bismuth inoculant effectively increases graphite nodule count and ferrite content, it was chosen for the late-stream (instant) inoculation step at 0.15 wt.%.
The effect of inoculation on nodule count (N) can be modeled as an exponential decay function of fading time, but a simplified relationship for immediate post-inoculation is:
$$ N \approx k \cdot I $$
where \( I \) is the inoculation amount and \( k \) is a constant dependent on inoculant type.
Experimental Results and Analysis
Samples from the three different nickel content melts (C1, C2, C3) were prepared for metallographic examination and mechanical testing according to standard procedures.
Microstructure Analysis
The samples were prepared by standard metallographic techniques including mounting, grinding, polishing, and etching. The microstructures were observed under an optical microscope. All three sample sets exhibited a nodular graphite structure with a nodularity grade ≥3 and a nodule size of 6-7 (ASTM standards). The matrix structure, however, varied significantly with nickel content.
The micrographs reveal the progressive change in matrix constituents. As nickel content increased, the area fraction of pearlite (the dark-etching constituent in the images, excluding graphite) increased markedly. Furthermore, at the highest nickel level (2.0 wt.%), the etchant revealed the presence of ledeburite, a hard and brittle eutectic mixture of austenite (transformed to pearlite and/or ferrite) and cementite. The quantitative results of the metallographic analysis are summarized in Table 3.
| Sample ID | Nickel Content (wt.%) | Pearlite Content (Area %) | Carbide/Ledeburite Content |
|---|---|---|---|
| C1 | 1.0 | 60 | <1% (Trace Carbides) |
| C2 | 1.5 | 70 | <1% (No Carbides) |
| C3 | 2.0 | 90 | ~8% (Ledeburite Present) |
The relationship between nickel content and pearlite fraction appears to be strongly positive. A linear regression can be fitted to the data points from C1 and C2 (excluding C3 where ledeburite formation alters the system):
$$ P\% = \alpha + \beta \cdot [\text{Ni}] $$
where \( P\% \) is the pearlite percentage, \( [\text{Ni}] \) is the nickel content in wt.%, \( \alpha \) is the intercept, and \( \beta \) is the slope. For the range 1.0-1.5 wt.% Ni, the slope is steep, indicating nickel’s potent effect on promoting pearlite.
The formation of ledeburite in sample C3 indicates that the composition entered a regime where the solidification path favors the metastable Fe-Fe3C system over the stable Fe-Graphite system, likely due to the combined effect of nickel and chromium suppressing graphitization and lowering the eutectic temperature. The critical nickel content for this transition in this specific high-silicon, chromium-containing nodular cast iron seems to be between 1.5 and 2.0 wt.%.
Mechanical Properties
Tensile test specimens were machined from the Y-blocks according to national standards, and hardness measurements were taken. The results of the mechanical tests are presented in Table 4. Each value represents an average from multiple tests, demonstrating good reproducibility.
| Sample ID | Ni (wt.%) | Tensile Strength, Rm (MPa) | Yield Strength, Rp0.2 (MPa) | Elongation, A (%) | Hardness, HBW |
|---|---|---|---|---|---|
| C1 | 1.0 | 665 | 510 | 7.0 | 223 |
| 657 | 500 | 7.5 | 221 | ||
| 663 | 508 | 7.0 | 229 | ||
| C2 | 1.5 | 691 | 583 | 5.0 | 258 |
| 694 | 579 | 5.5 | 257 | ||
| 682 | 581 | 5.0 | 251 | ||
| C3 | 2.0 | 716 | 620 | 2.0 | 270 |
| 701 | 627 | 2.5 | 276 | ||
| 717 | 631 | 2.5 | 274 |
The data clearly shows that, with other alloying elements constant, increasing nickel content leads to a continuous improvement in tensile strength, yield strength, and hardness. This is directly attributable to the increasing pearlite content and the solid solution strengthening provided by nickel in the ferrite/austenite (pearlite) matrix. The yield strength shows a particularly sensitive response to nickel addition.
However, a critical trade-off is observed with ductility. Elongation decreases progressively from about 7% at 1.0 wt.% Ni to 5% at 1.5 wt.% Ni. At 2.0 wt.% Ni, elongation plummets to approximately 2-2.5%, which falls below the specified minimum requirement of 3%. This severe loss in ductility is directly correlated with the appearance of ledeburite in the microstructure. Ledeburite, being a network of hard cementite, provides easy paths for crack propagation and severely limits plastic deformation, making the nodular cast iron brittle despite its high strength.
The relationship between strength and hardness for these nodular cast iron samples can be approximated by a linear law common for metallic materials. A correlation between yield strength and hardness (HB) might be expressed as:
$$ R_{p0.2} \approx c \cdot \text{HB} $$
where \( c \) is a material constant. For these alloyed nodular cast irons, the constant appears relatively stable across compositions without ledeburite.
The dramatic change in properties with nickel content highlights the importance of the pearlite/ferrite balance and the avoidance of brittle phases in engineering nodular cast iron. The optimal combination of strength and ductility for this specific high-silicon STQNiCr grade appears to be achieved at a nickel content of 1.5 wt.%.
Discussion on the Role of Nickel in Nodular Cast Iron
Nickel’s influence extends beyond simple phase promotion. In the context of high-silicon nodular cast iron, its effects are synergistic with other elements.
Solid Solution Strengthening: Nickel dissolves in both ferrite and austenite, causing lattice strain and increasing the stress required for dislocation motion. The strengthening contribution from solid solution can be described by an equation similar to:
$$ \Delta \sigma_{ss} = K_{Ni} \cdot [Ni]^{n} $$
where \( \Delta \sigma_{ss} \) is the increase in yield strength due to solid solution, \( K_{Ni} \) is a strengthening coefficient for nickel in iron, and \( n \) is often near 1 for dilute solutions.
Transformation Kinetics: Nickel lowers the eutectoid temperature and extends the transformation time for austenite to pearlite, allowing for a finer interlamellar spacing within pearlite. According to the Hall-Petch type relationship for pearlite, finer spacing increases strength:
$$ \sigma_y = \sigma_0 + k_y \cdot \lambda^{-1/2} $$
where \( \lambda \) is the interlamellar spacing and \( \sigma_0 \) and \( k_y \) are constants. Nickel promotes this refinement.
Graphite Morphology Stability: While nickel itself is not a graphitizer, it does not severely oppose graphitization like chromium. By stabilizing austenite, it may provide a more uniform matrix environment that helps maintain the spheroidal shape of graphite nodules during solidification and cooling, which is beneficial for the overall toughness of nodular cast iron.
Corrosion Mechanism: The enhanced corrosion resistance imparted by nickel is electrochemical. Nickel increases the nobility of the iron matrix. The passivation potential \( E_{pass} \) may shift to more active (less noble) values, making passivation easier. The corrosion current density \( i_{corr} \) in an active state is also reduced. The overall corrosion rate in an aqueous environment can be related to these parameters by the Stern-Geary equation for polarization resistance. The presence of nickel likely modifies the composition and protectiveness of the passive film, increasing its charge transfer resistance.
The complex interplay in this multi-component nodular cast iron system means that property optimization is a balancing act. The high silicon content aims for ferrite and corrosion resistance, chromium adds strength and some corrosion benefit but risks carbides, and nickel enhances strength and corrosion resistance but can promote pearlite excessively and, at high levels, lead to brittle constituents. The experimental data provides a clear window for the nickel content—around 1.5 wt.%—where the benefits are maximized and the drawbacks (low ductility, carbide formation) are minimized.
Conclusions
Based on the experimental investigation into the effect of nickel on high-silicon solid solution STQNiCr nodular cast iron, the following conclusions can be drawn:
- Using a charge composition of 60 wt.% pig iron, 15 wt.% steel scrap, and 25 wt.% returns, along with a chemical composition of 3.45-3.55 wt.% C, 3.30-3.50 wt.% Si, 0.65 wt.% Cr, and 1.5 wt.% Ni, followed by proper nodularizing (1.6 wt.% agent) and inoculation (1.25 wt.% total, including 0.15 wt.% Si-Bi inoculant) treatments, it is possible to produce STQNiCr nodular cast iron that meets the target mechanical properties (Rm ≥ 500 MPa, A ≥ 3%, carbides ≤ 1%).
- Increasing nickel content in the range of 1.0 to 2.0 wt.% causes a significant rise in the pearlite fraction of the matrix. At 1.0 wt.% Ni, pearlite is about 60%; at 1.5 wt.% Ni, it reaches 70%; and at 2.0 wt.% Ni, it attains approximately 90%. Furthermore, at the 2.0 wt.% Ni level, the microstructure begins to exhibit ledeburite, a brittle eutectic mixture, indicating a shift towards metastable solidification.
- The mechanical strength (tensile and yield strength) and hardness of the nodular cast iron increase monotonically with increasing nickel content, primarily due to the higher pearlite content and solid solution strengthening. However, ductility, as measured by elongation, decreases. The decline is gradual from 1.0 to 1.5 wt.% Ni but becomes drastic at 2.0 wt.% Ni, where elongation falls below 3%, failing to meet the specification. This is a direct consequence of the formation of ledeburite, which severely embrittles the material.
- Therefore, for this specific high-silicon STQNiCr nodular cast iron intended for applications requiring a balance of strength, ductility, and corrosion resistance, a nickel content of 1.5 wt.% is optimal. It provides a significant boost in strength and hardness over the 1.0 wt.% Ni variant while maintaining acceptable elongation and avoiding the formation of detrimental brittle phases. Higher nickel content, though potentially beneficial for corrosion resistance, compromises ductility unacceptably for many engineering applications.
This study underscores the critical role of precise alloying control in advanced nodular cast iron. Nickel is a powerful tool for tailoring the microstructure and properties of nodular cast iron, but its content must be carefully optimized in conjunction with other elements like silicon and chromium to achieve the desired performance profile for demanding environments such as marine engineering.