Fundamental Material Concepts for Downhole Tools: Grades, Materials, Designations and Cross-Reference Tables
This document summarizes the essential material knowledge required for the design and selection of downhole tools. It covers:
- The distinction between grade, material, and designation
- The main material families used in downhole tools
- The relationship between API grades and materials
- Cross-reference between AISI / GB / DIN / UNS systems
- Practical lookup tables for day-to-day engineering work
The focus is on steels and alloys commonly used in:
- Casing and tubing
- Downhole tools (packers, anchors, mills, fishing tools, completion tools, etc.)
- Sour (H₂S) and CO₂ environments governed by NACE MR0175 / ISO 15156
1. Core Concepts: Grade, Material, Designation
1.1 Material (Alloy System)
“Material” refers to the chemical composition and metallurgical system of a metal, as defined by standards or producers. It governs:
- Alloy family (carbon steel, low-alloy steel, martensitic stainless, austenitic stainless, duplex stainless, Ni-based alloys, etc.)
- Corrosion resistance (CO₂, H₂S, chloride, organic acids)
- Achievable strength and toughness levels
- Weldability, heat-treatment response, machinability
Typical examples:
- Carbon / low-alloy steels: AISI 4130, 4140, 4145, 4330V, GB 42CrMo
- Stainless steels: 13Cr, Super 13Cr, 22Cr, 25Cr, 304, 316L
- Ni-based alloys: Inconel 718, Incoloy 825, Incoloy 925, Alloy 725
The material answers the question: “What alloy system is this?”
1.2 Grade (Mechanical Strength Level)
In the API context (e.g., API 5CT, API 5D), a grade is essentially a mechanical strength level defined by standard, not a unique chemical composition.
For API 5CT casing/tubing, typical grades include:
- J55, K55, N80, L80, C90, T95, P110, Q125, etc.
A grade typically specifies:
- Yield strength range (e.g., 80 ksi for L80, 110 ksi for P110)
- Minimum tensile strength
- Required heat treatment (normalized, quenched and tempered, controlled rolling, etc.)
- Sometimes additional requirements on hardness, toughness, SSC resistance
The same underlying material, via different heat treatments, can meet different API grades. For example, a 4140/42CrMo alloy:
- At lower strength, can meet requirements similar to L80
- With more aggressive Q&T, can reach P110 or higher strengths
Thus, grade = performance level, while material = chemistry.
1.3 Designation (Name in a Specific Standard System)
A designation is the name/number assigned to a composition range in a particular standard system:
- AISI / SAE: 4130, 4140, 4145, 8620, 9310
- GB (China): 30CrMo, 42CrMo, 20CrNiMo, etc.
- DIN / EN (Europe): 25CrMo4, 34CrMo4, 20CrNiMo2, 36CrNiMo4, etc.
- UNS: G41300, G41400, S41000, N07718, etc.
- Trade names: Inconel 718, Incoloy 825, etc.
Different standard systems may assign different designations to the same or very similar composition ranges, which are often treated as equivalent or near-equivalent in engineering practice, subject to project standards and qualification.
1.4 Relationship Between the Three
Conceptually:
- Material defines the chemistry and metallurgy (e.g., 13Cr, 4140, Inconel 718)
- Heat treatment + material determines whether a given grade (e.g., L80, P110) can be met
- Designation is the name for that material within a specific standard system
Example from API 5CT L80:
- L80: grade (80 ksi yield)
- Sub-types:
- L80-1: Type 1 carbon steel
- L80-9Cr: ~9% Cr alloy
- L80-13Cr: ~13% Cr martensitic stainless
When only “L80” is written without “-1/-9Cr/-13Cr”, it is often implicitly referring to L80-1 (carbon steel) in many industry contexts, unless specified otherwise.
2. Overview of API 5CT Casing Grades (with Sour/Sweet Context)
2.1 Key Grades and Nominal Strength Levels
The table below summarizes widely used API 5CT grades with nominal yield strengths for quick reference (actual ranges are defined in the standard):
| Grade | Nominal Yield (ksi) | Nominal Yield (MPa) | Typical Application Context |
|---|---|---|---|
| H40 | 40 | ~276 | Shallow wells, low pressure |
| J55 | 55 | ~379 | Conventional oil & gas wells |
| K55 | 55 | ~379 | Slightly higher requirement than J55 |
| N80 | 80 | ~552 | Medium-deep wells (typically sweet) |
| L80 | 80 | ~552 | Sour wells & CO₂ environments |
| C90 | 90 | ~621 | High-pressure sour wells |
| T95 | 95 | ~655 | High-pressure sour & CO₂/H₂S wells |
| P110 | 110 | ~758 | Deep, high-pressure wells (sweet) |
| Q125 | 125 | ~862 | Ultra-deep, high-pressure wells |
Design work should always refer to the full API 5CT grade definitions rather than nominal values only.
2.2 L80 vs. L80-1, L80-9Cr, L80-13Cr
L80 is a grade; API 5CT further subdivides it by type (material family):
| Marking | Grade | Type | Material Family | Notes and Applications |
|---|---|---|---|---|
| L80-1 | L80 | Type 1 | Carbon steel | Most common carbon-steel L80 for sour service |
| L80-9Cr | L80 | Type 9Cr | ~9% Cr alloy | Enhanced CO₂ corrosion resistance |
| L80-13Cr | L80 | Type 13Cr | ~13% Cr martensitic SS | Widely used CRA for CO₂ wells with limited H₂S |
When “L80” is stated without the suffix, it is often interpreted as L80-1 in practice, unless the technical specification explicitly calls for 9Cr or 13Cr variants.
3. Main Material Families for Downhole Tools
For casing, grade is central. For downhole tools, the focus is more on material system + heat treatment + process control. The following families are used extensively in tool mandrels, housings, connectors, slips, and internal components.
3.1 Carbon and Low-Alloy Steels
3.1.1 Typical Designations and Cross-References
| AISI | GB (approx.) | DIN/EN (approx.) | UNS | Key Characteristics |
|---|---|---|---|---|
| 4130 | 30CrMo | 25CrMo4 | G41300 | Low-carbon Cr-Mo steel, good weldability, good toughness |
| 4140 | 42CrMo | 34CrMo4 | G41400 | Medium-carbon Cr-Mo steel, one of the main tool materials |
| 4145 / 4145H | — | — | G41450 | High-cleanliness 4140-type, used in drill collars, heavy tools |
| 4330V | 30CrNi2MoV | 34CrNi2MoV | K43300 | Ni-Cr-Mo-V high-strength steel, suitable for HPHT components |
3.1.2 Typical Performance and Applications
| Material System | Typical Yield Strength (Q&T) | Characteristics | Typical Tool Applications |
|---|---|---|---|
| 4130 / 30CrMo / 25CrMo4 | ~70–110 ksi | High toughness, good weldability | Pressure housings, cylinders, structural shells |
| 4140 / 42CrMo / 34CrMo4 | ~80–150 ksi | Balanced strength and toughness, widely available | Tool housings, subs, mandrels, milling tool bodies |
| 4145H | ~110–160 ksi | High cleanliness, good fatigue life | Drill collars, heavy-duty BHA components |
| 4330V | ~125–170 ksi | High strength + high toughness | HPHT tool bodies, high-load connectors |
3.2 Carburizing Wear-Resistant Steels (Slips, Wickers, Wear Parts)
| AISI | GB | DIN/EN | UNS | Typical Use |
|---|---|---|---|---|
| 8620 | 20CrNiMo | 20CrNiMo2 | G86200 | Slips, teeth, and wear parts after carburizing |
| 9310 | 18CrNiMo7 | 17CrNiMo6 | G93100 | Heavy-duty gears, kelly bushings, slip segments |
These steels are carburized and quenched to achieve surface hardness > 55 HRC while maintaining a tough core.
3.3 Stainless and Corrosion-Resistant Alloys (CRA)
3.3.1 Martensitic and Super Martensitic Stainless (13Cr Family)
| Common Name | UNS | GB (approx.) | DIN/EN (approx.) | Typical Notes |
|---|---|---|---|---|
| 13Cr (410-type) | S41000 / S42000 | 20Cr13, etc. | X20Cr13, etc. | L80-13Cr casing, CO₂-resistant tool parts |
| Super 13Cr | S41426, etc. | — | — | Enhanced resistance to CO₂ + chlorides |
Applications include CO₂ environments, selected sour service under NACE constraints, and CRA components for packers, valves, and completion tools.
3.3.2 Austenitic Stainless Steels
| Alloy | UNS | GB | DIN/EN | Typical Use |
|---|---|---|---|---|
| 304 | S30400 | 06Cr19Ni10 | X5CrNi18-10 | General purpose corrosion-resistant components |
| 316L | S31603 | 022Cr17Ni12Mo2 | X2CrNiMo17-12-2 | Better chloride resistance, some wetted components |
3.3.3 Duplex and Super Duplex Stainless Steels
| Common Name | UNS | GB | DIN/EN | Typical Use |
|---|---|---|---|---|
| 22Cr (2205) | S31803 / S32205 | 022Cr22Ni5Mo3N | X2CrNiMoN22-5-3 | Sour wells with H₂S + high Cl⁻, packer and housing parts |
| 25Cr (2507) | S32750 / S32760 | 022Cr25Ni7Mo4N | X2CrNiMoN25-7-4 | Offshore / extreme corrosive environments |
Duplex and super duplex steels require tight heat-treatment control to obtain a balanced ferrite/austenite ratio and avoid σ-phase embrittlement.
3.4 Ni-Based Alloys
| Trade Name | UNS | Typical CN Name | Features | Typical Applications |
|---|---|---|---|---|
| Inconel 718 | N07718 | GH4169 | High strength (120–150 ksi), excellent H₂S/Cl⁻/CO₂/high-T resistance | Critical packer connectors, safety valve internals, HPHT tools |
| Incoloy 825 | N08825 | — | Excellent resistance to H₂S + Cl⁻ + CO₂ | Valve bodies and housings in severe sour environments |
| Incoloy 925 | N09925 | — | High strength + high corrosion resistance | Critical components in sour gas tools, threaded connectors, etc. |
| Alloy 725 | N07725 | — | Higher strength and corrosion resistance vs. 718 | High-reliability connectors and fasteners in HPHT + severe sour |
Ni-based alloys combine high strength, outstanding corrosion resistance, and good high-temperature performance, at significantly higher cost and machining difficulty compared to steels.
4. Material and Grade Selection in Sour vs. Sweet Environments (NACE Perspective)
4.1 Carbon and Low-Alloy Steels
Under NACE MR0175 / ISO 15156, sour service conditions impose strict limitations, especially for SSC (sulfide stress cracking):
- Hardness for carbon and low-alloy steels is typically limited to ≤ 22 HRC (≈ 237 HBW) for SSC resistance.
- High-strength API grades like P110 and Q125 are generally not suitable for sour service unless special qualifications exist.
- Commonly used sour-service grades include:
- J55 / K55 / H40
- L80-1 (carbon steel L80)
- SSC-resistant versions of C90-2 and T95-2
Selection must consider H₂S partial pressure, temperature, pH, Cl⁻ concentration, and stress level, according to the detailed NACE clauses.
4.2 Stainless Steels and CRA
- 13Cr: suitable primarily for CO₂ environments; sour use is allowed only within certain H₂S, Cl⁻, temperature and pH limits.
- Super 13Cr: improved resistance for CO₂ + chlorides and moderate sour conditions.
- 22Cr / 25Cr duplex and super duplex: for H₂S + high Cl⁻ + moderate to high temperature conditions; widely used in offshore sour wells.
- Ni-based alloys (718 / 825 / 925 / 725): the preferred choice for extreme sour and HPHT environments where both corrosion and stress levels are critical.
5. Practical Lookup and Cross-Reference Tables
5.1 Common Downhole Tool Materials – Quick Reference
| Material / Designation | Family | Typical Yield Range* | Corrosion Resistance | Typical Uses | High-Level Environment Category |
|---|---|---|---|---|---|
| 4130 / 30CrMo / 25CrMo4 | Low-alloy steel | 70–110 ksi | Limited | Pressure housings, cylinders, structural shells | Sweet / slightly sour |
| 4140 / 42CrMo / 34CrMo4 | Low-alloy steel | 80–150 ksi | Limited | Tool housings, subs, mandrels, milling tool bodies | Sweet / slightly sour (hardness-controlled) |
| 4145H | Low-alloy steel | 110–160 ksi | Limited | Drill collars, heavy-duty BHA components | Sweet |
| 4330V | Ni-Cr-Mo high-strength | 125–170 ksi | Limited | HPHT tool bodies, critical high-load parts | HPHT |
| 8620 / 9310 | Carburizing steels | Tough core; surface >55 HRC | Limited | Slips, slip inserts, wickers, wear surfaces | Sweet |
| 13Cr / L80-13Cr | Martensitic stainless | 80–95 ksi | Good (CO₂ > H₂S) | CO₂-resistant casing, CRA internals, valve bodies | CO₂ wells, limited sour |
| Super 13Cr | Super martensitic SS | 95–120 ksi | Very good | CO₂ + chlorides + some H₂S, critical CRA tool parts | Sour / CO₂ / high-salinity |
| 22Cr (2205) | Duplex SS | 80–95 ksi | Very high | H₂S + high Cl⁻ packer components, CRA housings | Strong sour / high Cl⁻ |
| 25Cr (2507) | Super duplex SS | 90–110 ksi | Extremely high | Offshore and extreme corrosive tools | Offshore / extreme sour |
| Inconel 718 (GH4169) | Ni-based alloy | 120–150 ksi | Outstanding | Packer connectors, safety valve internals, HPHT parts | HPHT + severe sour |
| Incoloy 825 | Ni-based alloy | 70–90 ksi | Outstanding | Severe sour valve bodies, housings | Severe sour |
| Incoloy 925 | Ni-based alloy | 90–120 ksi | Outstanding | Sour gas tool connectors and critical load-bearing parts | Strong sour |
* Yield ranges are indicative engineering values. Design and qualification must rely on applicable standards, data sheets, and mill test certificates (MTC).
5.2 Major Overlapping Designation Systems
The following table highlights common cross-references between AISI, GB, DIN/EN, and UNS for typical downhole materials:
| Composition / Function | AISI / SAE | GB | DIN/EN | UNS | Notes |
|---|---|---|---|---|---|
| Low-alloy Cr-Mo steels | 4130 | 30CrMo | 25CrMo4 | G41300 | Used for housings, pressure barrels |
| Low-alloy Cr-Mo steels | 4140 | 42CrMo | 34CrMo4 | G41400 | Primary tool body and connector material |
| High-cleanliness Cr-Mo | 4145H | — | — | G41450 | Used for drill collars and heavy-duty tools |
| High-strength Ni-Cr-Mo | 4330V | 30CrNi2MoV | 34CrNi2MoV | K43300 | Used for HPHT tools and high-reliability parts |
| Carburizing wear steels | 8620 | 20CrNiMo | 20CrNiMo2 | G86200 | For slips, wickers, wear parts |
| Carburizing wear steels | 9310 | 18CrNiMo7 | 17CrNiMo6 | G93100 | For heavy-duty gears and slip segments |
| Martensitic stainless | 410 / 13Cr | 20Cr13 | X20Cr13 | S41000 | Basis for L80-13Cr and CO₂-resistant components |
| Duplex stainless | 2205 | 022Cr22Ni5Mo3N | X2CrNiMoN22-5-3 | S31803 / S32205 | Sour duplex steel for H₂S + Cl⁻ |
| Super duplex stainless | 2507 | 022Cr25Ni7Mo4N | X2CrNiMoN25-7-4 | S32750 / S32760 | Offshore and extreme sour conditions |
| Ni-based alloy | — | GH4169 | — | N07718 | Inconel 718 |
| Ni-based alloy | — | — | — | N08825 | Incoloy 825 |
| Ni-based alloy | — | — | — | N09925 | Incoloy 925 |
These mappings are approximate and should be verified against the relevant standards and project specifications when used for equivalency.
6. Use in Engineering Practice
In practical downhole tool design and material selection, the following principles generally apply:
- For casing and tubing, API grade (J55, N80, L80, P110, etc.) defines the mechanical properties; the underlying material (chemistry) may vary within the standard’s allowable ranges and must be confirmed via MTC.
-
For downhole tools, the starting point is the material system (e.g., 4140, 4330V, 13Cr, 22Cr, 718), followed by:
- Heat-treatment design (strength level, hardness limits for NACE, toughness margins)
- Corrosion environment (sweet vs. sour, CO₂, Cl⁻, temperature, pH)
- Manufacturing route (forging, machining, carburizing, surface hardening, etc.)
All final selections should be checked against:
- Applicable standards (API 5CT/5D, ASTM, EN, GB, NACE MR0175 / ISO 15156)
- Project-specific requirements and load envelopes
- Mill Test Certificates and, where applicable, qualification test data
This framework is intended as a persistent reference for engineers working on casing, completion, and downhole tool systems, and as a basis for internal standards, training material, and technical appendices in design and procurement documents.
