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Metals 2017, 7(9), 351;

Study of 13Cr-4Ni-(Mo) (F6NM) Steel Grade Heat Treatment for Maximum Hardness Control in Industrial Heats

Dipartimento di Ingegneria Civile e Industriale, Università di Pisa, Largo Lucio Lazzarino, 2, 56122 Pisa, Italy
Nuovo Pignone Tecnologie Srl, BHGE, Via Felice Matteucci, 2, 50127 Firenze, Italy
Dipartimento di Ingegneria Industriale, Università di Tor Vergata, Via del Politecnico, 1, 00133 Roma, Italy
Author to whom correspondence should be addressed.
Received: 16 June 2017 / Revised: 30 August 2017 / Accepted: 31 August 2017 / Published: 6 September 2017
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The standard NACE MR0175 (ISO 15156) requires a maximum hardness value of 23 HRC for 13Cr-4Ni-(Mo) steel grade for sour service, requiring a double tempering heat treatment at temperature in the range 648–691 °C for the first tempering and 593–621 °C for the second tempering. Difficulties in limiting alloy hardness after the tempering of forged mechanical components (F6NM) are often faced. Variables affecting the thermal behavior of 13Cr-4Ni-(Mo) during single and double tempering treatments have been studied by means of transmission electron microscopy (TEM) observations, X-ray diffraction measurements, dilatometry, and thermo-mechanical simulations. It has been found that relatively low Ac1 temperatures in this alloy induce the formation of austenite phase above 600 °C during tempering, and that the formed, reverted austenite tends to be unstable upon cooling, thus contributing to the increase of final hardness via transformation to virgin martensite. Therefore, it is necessary to increase the Ac1 temperature as much as possible to allow the tempering of martensite at the temperature range required by NACE without the detrimental formation of virgin martensite upon final cooling. Attempts to do so have been carried out by reducing both carbon (<0.02% C) and nitrogen (<100 ppm) levels. Results obtained herein show final hardness below NACE limits without an unacceptable loss of mechanical strength. View Full-Text
Keywords: supermartensitic stainless steels; tempering treatments; austenite reversion supermartensitic stainless steels; tempering treatments; austenite reversion

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De Sanctis, M.; Lovicu, G.; Buccioni, M.; Donato, A.; Richetta, M.; Varone, A. Study of 13Cr-4Ni-(Mo) (F6NM) Steel Grade Heat Treatment for Maximum Hardness Control in Industrial Heats. Metals 2017, 7, 351.

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