The thermal specification (also known as TCase) for your Intel® Core™ i5-3450 Processor is 67.4 °C: http://ark.intel.com/products/65511/Intel-Core-i5-3450-Processor-(6M-Cache-up-to-3_50-GHz)
The TCase is a number established by Intel® as a point of reference in order to understand what could be expected as per normal processor temperature.
Anything from the Tcase and below will be the expected temperature of the processor in normal use, anything that doesn't stress out the processor (watching movies, burning CDs, browsing the internet, creating documents, etc.) When the processor is stressed out meaning that you are running heavy processor applications that take control of the CPU or uses it at 100% the temperature will go beyond the Tcase. It can perfectly reach 80 to 85 degrees and the processor will still be OK. The cooling fan is in charge to keep that temperature there.
If the processor temperature reaches 100 degrees or more it will send a signal to the motherboard to shut down to prevent mayor damages and most likely it won't be possible to turn the computer back in until it cools down.
The normal processor temperature will depend on the chassis type, the hardware involved and the location of the computer, and it usually is lower than the Tcase.
Intel® processors have been validated and tested using the respective stock cooling solution it was designed to work with, the use of third party cooling solutions (Thermal Interface Material and heat sink fan) are not supported not covered by the 3 year limited warranty,
If needing further technical assistance, feel free to contact your local support group during business hours: http://www.intel.com/p/en_US/support/contact/phone