Control of Factors Influencing the Mechanical Properties of AISI 4130 Material

Abstract

AISI 4130 is widely used in wellhead equipment and oil tree equipment due to its high strength, good low-temperature impact properties, resistance to hydrogen sulfide-induced stress corrosion in acidic environments, and favorable price-to-performance ratio. Different strength properties can be achieved through varying control methods during steel-making, forging, and heat treatment. This article discusses the control of material selection, raw materials, and heat treatment to achieve stable properties of AISI 4130 at 75K and higher grades.

Keywords

AISI 4130; mechanical properties; heat treatment

Introduction

AISI 4130 material is extensively used in wellhead installations and oil tree equipment due to its high strength, excellent low-temperature impact performance, resistance to hydrogen sulfide-induced stress corrosion in acidic media, and cost-effectiveness. AISI 4130 typically comes in four strength grades: 60K, 75K, 80K, and 85K. For wellhead and oil tree equipment rated above 69.0 MPa (10,000 psi), materials of 75K grade or higher are commonly employed to meet performance requirements while reducing product weight and costs.

This article explores the factors influencing the mechanical properties of AISI 4130 from three perspectives: material selection, raw material control, and heat treatment control.

1. Material Selection

In selecting materials, it’s essential to satisfy ER ≤ DI, meaning the product dimensions should not exceed the quenching depth of the alloy used. This ensures uniform quenching throughout the cross-section, meeting the required mechanical properties. According to API 6HT standards, which supplement the heat treatment details required by API specifications, improper heat treatment of critical components or large cross-sectional parts is one of the primary causes of field failures. This is particularly relevant since the small QTC specimens allowed by API 6A may not represent the mechanical properties of larger equivalent circular sections. Therefore, if the part design does not exceed the quenching diameter of the material used, the standard test block specified in API 6A with ER = 5 can be used. If greater quenchability is required, larger test blocks or samples taken from the body extension should be utilized.

In summary, the choice of alloy materials primarily depends on the geometry and performance requirements of the part. If the part undergoes heat treatment in the forged, rolled, or cast state, it should be roughly machined before the final heat treatment to ensure maximum quenchability.

2. Control of Raw Materials

2.1 Chemical Composition Control

The chemical composition of raw materials includes basic elements such as C, Si, Mn, Cr, and Mo. The principle is that these elements should guarantee sufficient quenchability and hardenability of the material. The ideal quenching diameter (DI) of AISI 4130 material can be calculated using the formula given in ASTM A255: DI = (0.54C) * (1 + 3.33Mn) * (1 + 0.36Ni) * (1 + 0.7Si) * (1 + 2.16Cr) * (1 + 3Mo)

Additionally, controlling the levels of S and P is crucial. The sulfur (S) content should ideally be kept below 0.005%, with a maximum limit of 0.15% in technical specifications. Increased S content lowers the material’s fracture toughness (KIC) and increases the impact transition temperature (ITT). The relationship between S and KIC is shown in Figure 1, and the effect of sulfides on ITT is illustrated in Figure 2.

For phosphorus (P), which relates to temper embrittlement, maintaining P levels below 0.015% is advisable. Aluminum (Al) can refine grain structure when present in the steel, with an optimal Al content range of 0.015% to 0.03%. However, excessive Al can form harmful oxide inclusions.

Other elements like Cu, Sn, Sb, and As are generally controlled as impurities. Copper (Cu) tends to increase brittleness, whereas adding an appropriate amount of Nickel (Ni) can counteract this effect, with a preferred Cu:Ni ratio of 1:2.

2.2 Control of Segregation in Raw Materials

Segregation within the steel leads to non-uniformity in microstructure and properties. To control segregation, the focus is on the production process at the mill. For ingots with a taper (larger at the top and smaller at the bottom), the mold angle is typically set between 6° and 8°. For ingots with a reverse taper (smaller at the top and larger at the bottom), the mold angle is usually between 2° and 5°.

3. Heat Treatment Control

3.1 Control of Heat Treatment Cooling Capacity

Proper heat treatment, including quenching cooling rates, is crucial. Diffusion annealing (normalizing) or increasing the quenching cooling rate can reduce the impact of segregation on material properties. Hydrogen (H) content should be controlled below 1.6 to 2 PPM, oxygen (O) content below 20 to 30 PPM, and nitrogen (N2) content below 30 PPM, ideally around 40 to 50 PPM.

3.2 Control of Heat Treatment Process Parameters

Controlling parameters such as soaking time and furnace loading is necessary for each product’s unique heat treatment process. Adequate spacing between products (at least 75 mm according to AMS 2759 standards) ensures uniform heat treatment atmosphere circulation. Additionally, the water temperature during quenching should ideally be maintained between 15 to 20°C.

Conclusion

Based on the analysis of factors influencing the mechanical properties of AISI 4130, the following controls are recommended to achieve consistent properties at the 75K level and above:

  1. Design and control product shape and size based on the quenchability of AISI 4130 to satisfy ER ≤ DI, ensuring uniform quenching.
  2. Limit sulfur (S) content to within 0.15%, phosphorus (P) content to below 0.015%, and maintain aluminum (Al) content between 0.015% to 0.03%. Optimal Cu:Ni ratio should be 1:2.
  3. Utilize diffusion annealing (normalizing) or increase the quenching cooling rate to mitigate the effects of segregation on material properties.
  4. Maintain hydrogen (H) content below 1.6 to 2 PPM, oxygen (O) content below 20 to 30 PPM, and nitrogen (N2) content below 30 PPM.
  5. Ensure adequate spacing between products during heat treatment (minimum 75 mm) and maintain quenching water temperature between 15 to 20°C.