1. Question: For a welded pipe application requiring excellent corrosion resistance in addition to high strength, which of the three grades, Q390, Q420, or Q460, would be the most cost-effective starting point?
Answer: None of these base grades offer inherent "stainless" level corrosion resistance. Their corrosion resistance is similar to plain carbon steel and relies on the formation of a protective patina. However, for improved corrosion resistance, they can be used in a "weathering steel" formulation (often with added Cu, P, Cr, and Ni). Starting from a lower strength like Q390 would be more cost-effective, as the alloy budget for corrosion resistance can be added without exceeding the cost of a very high-strength base like Q460. If high strength and high corrosion resistance are both required, a modified Q420NQR (a weather-resistant grade) is a more common and balanced choice than Q460.
2. Question: What is the typical difference in price and availability between Q390 and Q460 welded pipes in the current market?
Answer: Q460 welded pipes are generally more expensive and less readily available than Q390 pipes. The higher cost is due to the more complex alloying design (higher Mn, plus Nb, V, Ti), stricter processing controls (precise rolling and cooling), and often lower production yields. Q390 represents a more common, "entry-level" high-strength material, making it more widely stocked by distributors and available for quicker delivery. Q460 is often a "make-to-order" product for specific projects requiring its ultimate strength, resulting in a higher price and longer lead times.
3. Question: How does the flattening test verify the quality of the longitudinal weld seam on a Q420 welded pipe?
Answer: The flattening test is a simple but powerful destructive test. A ring sample cut from the Q420 pipe is placed between two parallel plates. The plates are pressed together to flatten the ring until the distance between the plates reaches a specified value, typically related to the pipe's outside diameter and wall thickness (e.g., H = (1+e)*t / (e+D/t)). For a valid Q420 pipe, the flattened sample must not show any cracks or flaws on the inside, outside, or end surfaces until the plates have reached the specified distance. If a weld defect, lack of fusion, or brittle zone is present, it will cause a crack to open up under this extreme deformation, failing the test.
4. Question: For a highly restrained welded joint on a thick-walled Q460 pipe, what combination of preheating, interpass temperature, and PWHT is typically specified?
Answer: For such a demanding application, a stringent thermal procedure is mandatory. First, a high preheat of 120-150°C is applied to the entire joint area. Second, the interpass temperature (the temperature of the steel between successive weld passes) must be maintained, typically not falling below the preheat temperature, e.g., 150-250°C, to ensure slow cooling. Immediately after welding, the joint is subjected to a post-heat or "hydrogen bake-out" at 250-300°C for a minimum of 2 hours to diffuse out hydrogen. Finally, a full Post-Weld Heat Treatment (PWHT) stress-relief anneal is performed, heating the entire joint to 600-650°C, holding for one hour per inch of thickness, then slowly cooling.
5. Question: What is the significance of the "upper yield strength" (ReH) versus "lower yield strength" (ReL) in the revised standards for Q390, Q420, and Q460, and why was this change made?
Answer: Older standards specified the lower yield strength (ReL), which is the stress at which a material yields and begins to deform plastically after the initial yield point drop. The revised standards have switched to specifying the upper yield strength (ReH), which is the peak stress just before this initial drop occurs. This change was made to align Chinese steel standards with most international standards (like ISO 6892). ReH is a more consistent and reproducible value to measure, especially for high-strength steels like Q460 that may not exhibit a clear, discontinuous yielding behavior. It provides a clearer, more standardized measure of the onset of plastic deformation.
