Immersion quenching is one of the most widely used processes for achieving martensitic and bainitic steels. A comprehensive modeling treatment of quenching requires a description of the surface heat flux or heat transfer coefficient. Generally, the heat transfer coefficients obtained during the quenching of a material not undergoing a phase change, such as austenitic stainless steel, are used for calculating the phase change in an alloy steel also. In order to accurately model phase transformation, one must characterize the heat transfer process specific to the quenchant–steel combination in question. This work reports the development of numerical models for the simultaneous estimation of surface heat flux, austenite decomposition, and hardness during the immersion quenching of carbon and alloy steels in plant conditions. The algorithm couples non-linear transient inverse heat transfer with phase transformation, resulting in heat flux values specific to the steel grade–quenchant combination in actual practice. The effects of the soaking temperature, component surface conditions, quenchant conditions, plant operating practices, and so on can be addressed satisfactorily with this method. The austenite decomposition models use a unique approach consistent with both the time–temperature–transformation diagram of the steel and Fe-C equilibrium phase diagrams. Portable and self-contained handheld equipment was designed for testing in the plant. The equipment was used for computing the surface heat flux at the mid-section of a cylindrical specimen of medium carbon 1050 grade steel (25 mm in diameter by 100 mm in length) quenched in an aqueous solution of a polymer. Using the transient heat flux values, the microstructure evolution and hardness across the cross section of the specimen were simultaneously computed. The hardness profile and the microstructure distribution across the specimen section are presented and are corroborated by laboratory measurements. It was found that in this specific case of polymer quenching, the surface hardness was lower than the core hardness due to an anomalous heat transfer condition, which is explained via the use of the models developed in this article.