Metallurgical and Materials Transactions B | 2021
Discussion of “Prevention of High-Temperature Surface Degradation in SiMo Cast Irons by Cr and Al Alloying”
Abstract
INTERNAL combustion engines will persist for some time. Ferritic cast iron continues to be an important alloy for automotive exhaust manifolds and turbocharger housings due to their relatively low cost as compared to Ni-resist iron, ferritic stainless steel, and austenitic stainless steel. In terms of chemical composition (weight pct used throughout the letter), this group of ferritic cast iron in production mainly includes silicon molybdenum (SiMo), and its variants such as SiMoCr, SiMoAl, SiMoV, SiMoW and other formulas. Recently, the oxidation testing of SiMo, SiMoCr, and SiMoAl specimens and subsequent SEM and TEM analysis of the oxide scales were published in the article by Lekakh et al., referred to hereafter as article 4. The dimensions of the small specimens are 35 9 14 9 5 mm. The specimens were isothermally tested in a tube furnace at the temperatures of 650 C, 700 C, 750 C and 800 C respectively, for 100 hours. Article 4 concluded from the static oxidation testing that the maximum working temperatures of SiMo, SiMoCr, and SiMoAl irons were 700 C, 750 C, and 800 C, respectively. This letter attempts to comment on, and complement, the findings on the working temperatures of SiMo based cast irons reported in article 4 for automotive exhaust component applications. Hot oxidation resistance and forming protective oxide scales are certainly critical for elevated-temperature applications. However, exhaust components can experience over the range of operating temperatures, under conditions of simultaneously varying thermal and mechanical loads. Therefore, it can be misleading when determining the temperature capability relying only on the static (unconstrained) oxidation testing, and assigning the oxidation resistance ratings to the various alloys. Comprehensive investigations of cast ferrous alloys were conducted in Wescast Industries for exhaust component applications. The evaluation matrix consisted of chemical composition, microstructure, mechanical, hot oxidation (static and cyclic testing), dimensional stability, thermophysical, manufacturability (foundry process, heat treatment if required, and machining), and thermal cycling testing. With regard to the thermal cycling testing, two types of specimens were employed to assess thermal durability and working temperature: small cylindrical (or disk) specimens and exhaust manifolds. The thermal cycling testing of manifolds is briefly introduced in this letter. An apparatus called as engine exhaust simulator (ESS) was commissioned using natural gas burners to simulate the engine dynamometer testing of manifolds and turbocharger housings, as displayed in Figure 1. Exhaust manifolds and Y-block castings were made of SiMo4 (4.0 pct Si) and SiMo5 (5.0 pct Si) iron respectively, and then went through the above-mentioned evaluation matrix. As an example, a thermal cycling profile was adopted for the EES testing of SiMo4 and SiMo5 manifolds: (1) 870 oC for the maximum exhaust gas temperature, and (2) 5 min for heating segment and 5 min for cooling segment under forced air condition (cooling to ~ 130 oC), i.e., 10 minutes for one thermal cycle. In these tests, the outer skin temperatures of the manifold were measured using thermocouples on multiple locations. The peak metal temperature was 760 oC during the EES testing. Thermal cycle counts of up to 3000 were achieved for SiMo4 parts and then the testing was halted as a standard design specification. This corresponded to more than two months of running on the EES test apparatus. Surprisingly (at that time), the SiMo5 manifolds lasted only 30 pct of the thermal cycle specification (3000 cycles) required, and prematurely cracked—probably due to lacking of ductility. DELIN LI is with the CanmetMaterials, Natural Resources Canada, 183 Longwood Road South, Hamilton, ON L8P 0A5, Canada. Contact e-mail: [email protected] Manuscript submitted December 6, 2020; accepted February 9, 2021.