1. **What type of steel is P91?**
* P91 (ASTM A335 P91, ASTM A213 T91, etc.) is a **high-temperature low-alloy steel**. It is specifically a **chromium-molybdenum-vanadium strengthened ferritic/martensitic steel** designed for high-temperature service in power plants and petrochemical industries. Its key characteristic is excellent creep strength and oxidation resistance at elevated temperatures (up to around 600°C / 1112°F). While its Chromium content (~8-9.5%) is higher than many traditional low-alloy steels, it is still generally classified within the low-alloy steel family for high-temperature applications.
2. **What are considered low-alloy steels?**
* Low-alloy steels (LAS) are steels whose total intentional alloying element content (excluding carbon) is generally less than **5% by weight**, but typically ranges between **1% to 4%**. Common alloying elements added in significant amounts include **Manganese (Mn), Chromium (Cr), Nickel (Ni), Molybdenum (Mo), Vanadium (V), Silicon (Si), and sometimes Boron (B)**. The primary purpose of these additions is to improve mechanical properties like strength, toughness, hardenability, and wear resistance compared to plain carbon steels, often without requiring heat treatment or enabling better performance after heat treatment. They are widely used in structural applications, pressure vessels, pipelines, machinery, automotive components, and military hardware.
3. **Is B7 low alloy steel?**
* **Yes, ASTM A193 Grade B7 is classified as a low-alloy steel.** It is a medium carbon (typically ~0.38-0.48% C), chromium-molybdenum steel (typically ~0.75-1.20% Cr, ~0.15-0.25% Mo). Its alloy content falls well within the typical range for low-alloy steels (total alloying elements < 5%). B7 is specifically designed for high-strength bolting applications for pressure vessels, valves, flanges, and fittings in elevated temperature service.
4. **How to identify low alloy steel?**
* Identifying low alloy steel definitively requires knowing its composition. Methods include:
* **Material Test Report (MTR/C of C):** The most reliable way. This document from the steel mill lists the actual chemical composition.
* **Positive Material Identification (PMI):** Using portable X-ray Fluorescence (XRF) or Optical Emission Spectrometry (OES) guns to measure the elemental composition on-site or in the lab.
* **Spark Testing (Less Reliable):** Observing the sparks produced when grinding the steel. Low-alloy steels typically produce medium-length, moderately forked orange-yellow sparks compared to the long, straight straw-colored sparks of low-carbon steel or the short, red sparks with little bursting of high-carbon/tool steels. However, this requires significant experience and is not precise.
* **Laboratory Chemical Analysis:** Wet chemistry or instrumental methods (like ICP-OES) performed in a lab provide the most accurate composition.
* **Visual & Application Context (Indicative Only):** While unreliable, high-strength structural shapes, heavy plates, pressure vessel components, high-strength bolts, or parts known for toughness/wear resistance often suggest low-alloy steel, especially if they look different (e.g., darker mill scale) than mild steel.
5. **What are the ASTM grades of low alloy steel?**
* ASTM has numerous grades covering low-alloy steels for different applications. Some prominent categories and examples include:
* **High-Strength Structural Steels:** ASTM A572 (Grades 42, 50, 55, 60, 65 - Columbium-Vanadium steels), ASTM A588 (Weathering steels), ASTM A514/A517 (Quenched & Tempered high-yield strength).
* **Pressure Vessel Steels:** ASTM A387 (Cr-Mo plates - e.g., Gr 2, 5, 11, 12, 21, 22), ASTM A542/A542M (pressure vessel quenched & tempered Cr-Mo plates), ASTM A302/A302M (Mn-Mo & Mn-Mo-Ni plates).
* **High-Temperature Service Piping/Tubing:** ASTM A335 (Seamless Ferritic Alloy-Steel Pipe - e.g., P5, P9, P11, P22, P91), ASTM A213 (Seamless Ferritic and Austenitic Alloy-Steel Boiler Tubes - e.g., T5, T9, T11, T22, T91).
* **Bolting Materials:** ASTM A193 (Alloy-Steel and Stainless Steel Bolting for High Temperature Service - e.g., B5, B6, B7, B16), ASTM A320 (Alloy-Steel Bolting Materials for Low-Temperature Service - e.g., L7, L43).
* **Bearing Steels:** ASTM A295 (High-Carbon Anti-Friction Bearing Steel).
* **Other:** ASTM A506 (Alloy Steel Strip), ASTM A507 (Alloy Steel Sheet/Strip), various grades within specifications like A829 for plates. *(Note: This is not exhaustive; many specific grades exist within these and other ASTM specs).*
