The action will take place on 25 January and 1 February and affect Neath Port Talbot Hospital, Morriston in Swansea and Princess of Wales in Bridgend.
Unison said non-emergency surgery was "likely to be severely disrupted".
The health board said it was fully committed to resolving concerns.
Unison said affected members at Abertawe Bro Morgannwg University Health Board wanted "pay parity with colleagues doing the same job in other hospitals in Wales".
Earlier this month, 54 staff members voted in favour of taking the industrial action, with two voting no, the turnout for the ballot was 78.9%.
Mark Turner, Unison organiser for ABMU, added: "They are highly skilled technicians of many years training yet are treated as though they have just walked in off the street.
"They are extremely disappointed that after two years of discussions the health board continues to ignore their very reasonable claim in the hope they will go away."
The health board said all staff were valued and it had been working with the trade union to develop a new job description.
The board said the current service needed a workforce with a mixture of skills at different bands.
"We are sorry our continuing discussions with Unison have not been able to resolve our staff's concerns," a spokesman added.
"We remain fully committed to continuing our partnership work with Unison to resolve these concerns and have offered to involve ACAS to assist with this issue.
"In the meantime, we would like to reassure patients should strike action go ahead we will be working to ensure any disruption to services are minimal."
A similar dispute involving X-ray department workers has been resolved.
Staff in the sterilisation and disinfection units at some hospitals in south Wales will go on strike for two days over equal pay.