By Prashant Poojary, Ferro Tech India Pvt Ltd.
Emission regulations for vehicles have become increasingly stringent with the proposed nation-wide adherence to Bharat Stage-VI (BS-VI) emission norms by 2020. This decision will have cost implications for oil refineries as well as auto manufacturers to upgrade technology.
Currently BS-IV fuel is being supplied to 13 cities, including NCR, while the rest of the nation still gets BS-III fuel. In conformance with BS-VI, refineries will have to provide fuel with sulphur content of 10 ppm as against the current limit of 50ppm in BS-III (80% reduction). Similarly, auto manufacturers will have to upgrade technology to comply with BS-VI diesel NOx emission limits of 0.08 gms/km as against 0.5 gms/km in BS-III (84% reduction) and particulate matter emission limits of 0.0045 gms/km versus 0.05 gms/km (91% reduction). For gasoline engines, the BS-VI NOx emission limit is 0.06 gms/km against 0.08 gms/km in BS-III (25% reduction).
Type 409 stainless steel in exhaust systems
In the 1960s, the US pioneered the usage of type 409 ferritic stainless steel in exhaust systems. Advent of type 409 stainless steel in Japan began with the intensifying regulations over exhaust gas in the 1970s. The Indian Government had made it mandatory for all cars to conform to the Bharat Stage II norms by April 2005. This gave a big boost to usage of type 409 stainless steels in exhaust system. Type 409 stainless steel is ferritic steel with 11% chromium. Additionally, the carbon content is kept below 0.03%, and microalloying addition of niobium (Nb) and/or titanium (Ti) is carried out to tie up interstitial elements like carbon and nitrogen by formation of precipitates (stabilization).
Current scenario
The exhaust components are broadly classified into two categories: hot-end and the cold-end. The hot-end exceeds temperature of 550°C and comprises of manifold and catalytic convertors. The cold-end is subjected to service temperature below 550°C and comprises of resonator, muffler and tail pipe.
Stainless steel for both the hot-ends and cold-ends are sourced from local manufacturers as well as imports. Domestic stainless steel plants supply 441 (Nb-Ti) grade to OEMs fabricating the hot-end portion of exhaust system. Dual stabilized variants of 441, 436, 439, 429, 432, 430, 443 and 444 are being imported for hot-end portion.
The OEMs fabricating cold-end of the exhaust system use 409-Ti from domestic stainless steel manufacturers and imports. The market demand for 409-Ti exhaust components stood at over 80,000 metric tonnes in FY 2015-16. Around 30 per cent of this demand was met by imports and rest was catered by domestic stainless steel manufacturers.
Property demands for the cold-end portion of exhaust system
The cold-end portion of exhaust system is primarily subject to corrosion failure both pitting and intergranular corrosion. Additionally, it can also be subject to fatigue loads at higher temperature, especially in gasoline vehicles where exhaust gas temperatures in the resonator approaches 600°C. The complex shapes of the exhaust components demand good drawability.
With increasingly demanding service conditions in exhaust components, in tune with the upcoming stringent emission norms, there is a growing need to focus on cost-effective Nb-Ti dual stabilized 409 stainless steel. This article will compare dual stabilized 409Nb-Ti stainless steel with conventionally used 409-Ti and highlight it’s advantages with respect to corrosion resistance, deep drawability, high temperature properties and weldability.
Corrosion resistance and formability
Cyclic corrosion test is a standard technique to determine corrosion resistance of steels used for automotive mufflers. A lot of corrosive anions exist in the exhaust condensate. 409Nb-Ti and 409-Ti were subjected to exhaust gas condensate corrosion test as seen in Figure 1.
Life prediction was obtained assuming constant corrosion rate and relationship function between the corrosion time and pitting depth by the least square method. The life of two stainless steel 409Nb-Ti and 409-Ti was calculated for a maximum pitting depth of 1mm. Based on the above relationship, 409Nb-Ti has 40% longer lifespan compared to 409-Ti(1).
Researchers at Allegheny Technologies have compared 409Nb-Ti with 409-Ti to check for evidence of intergranular corrosion on the TIG welds and/or weld heat affected zones. Among 36 different test coupons of 409-Ti, 11 test coupons were dual stabilized 409 Nb-Ti. All test coupons of 409Nb-Ti prevented sensitization resulting in superior corrosion resistance compared to 409-Ti(2).
Higher r-bar values in steel indicate higher drawability and lower ?r values imply better isotropy. 409Nb-Ti has higher r-bar value (1.35) and lower ?r value (0.76) compared to 409Ti (1.21 and 0.79 respectively).
High temperature properties
The typical working range for exhaust components vary from ~900°C in manifold to ~150°C in the tail pipe. The high temperature yield strength can be seen in Figure 2. The yield strength of 409Nb-Ti is higher than 409-Ti, especially at the temperature below 700°C. The wide working range of temperature of exhaust components can cause thermal fatigue failure. S-N fatigue curves for the 409-Ti and 409Nb-Ti (0.15%Nb) at 600p C was studied. The high temperature fatigue of 409Nb-Ti was superior to 409-Ti reiterating the beneficial effects of niobium in type 409 stainless steel (1).
Weldability
409Nb-Ti is readily weldable using conventional welding procedures which include high frequency, resistance, gas tungsten arc (GTAW or TIG), gas metal arc (GMAW or MIG), electron beam and laser techniques. Autogenous welds are common, especially in tubing. Filler wire conforming to AWS standard A5.9 EC409Cb is used for welding 409Nb-Ti components. Austenitic stainless steel filler wire rods can be used to weld 409Nb-Ti to itself, carbon steel or austenitic stainless steels (2).
Current usage of 409Nb-Ti
Baosteel has successfully implemented usage of 409Nb-Ti as a better material to imported 409-Ti. 409Nb-Ti from Baosteel has passed five stringent certification processes for General Motors, i.e., material level, manufacturing test, system level test, vehicle level test and after-sales material corrosion life expectancy assessment. Certification of Baosteel 409Nb-Ti was followed by full material switching in GM Excelle and Cruze. Silencer samples made of 409Nb-Ti and imported 409-Ti stainless were field-tested in six cities in China and were extracted for evaluation of corrosion. A test analysis by Shanghai University showed that parts made of Baosteel dual stabilized stainless steel had superior corrosion resistance than parts made of imported 409-Ti stainless steel (3).
Way forward
The proposed Bharat Stage-VI fuel emission norms will be mandatory by 2020 pan-India. In view of stricter emission controls, both the fuel quality and the vehicle-exhaust play a pivotal role. Petroleum refineries have begun huge investments to adhere to the strict sulphur levels in the fuel. On the part of the auto-manufacturers, the fuel economy of internal combustion engines and the cleanliness of the associated emissions can be improved by implementing various technologies.
The main function of vehicle exhaust is to accelerate transformation of toxic components such as CO, NOx into non-toxic components. Higher transformation efficiency calls for higher exhaust gas temperatures. Consequently, the design of emission system components necessitate a new generation of materials that can meet the high demands for improved emission reduction.
Summarizing the aforementioned benefits of 409Nb-Ti compared to 409-Ti like superior exhaust gas condensate corrosion resistance, higher life cycle, superior intergranular corrosion, good drawability, high temperature strength, higher fatigue resistance and weldability. Dual stabilised ferritic stainless steels like 409Nb-Ti represent the cost-effective solution for exhaust system components for the immediate future.
Acknowledgements
The author would like to thank CBMM Technology Suisse SA and Ferro Tech India Pvt. Ltd. for granting permission to publish the article.
References:
- Chen, E.; Wang, X.; Shang, C.: A low cost effective ferritic stainless steel microalloyed by higher Nb for automotive exhaust system. Proceedings of HSLA 2015. Hangzhou, China.
- Allegheny Technologies Incorporated. ATI 409HP. Accessed: [1st Nov. 2016].
3. Baosteel Group Corporation. Media Center (2011, August 18). Three Kinds of Baosteel Stainless Steel Enter GM Global Procurement List.